There’s no shock quite like one from somebody you were sure you knew inside out. When my best friend of more than 30 years told me she was having an affair, I burst out laughing, assuming it was a joke. How could it be anything else?We messaged all day every day, a constant, endless conversation where we consulted each other on everything, from what to have for lunch to whether to increase our HRT prescriptions.I would have bet my house that I knew everything about Sophie – and had done since the day we met at university. It had been the friendship equivalent of love at first sight and, since then, we had shared every detail of our lives. Or so I had thought.Now, a bombshell out of the blue. She was sleeping with a man who wasn’t her husband; a guy she had met through work, whose attention and desire had invigorated her in ways she couldn’t have imagined before. It had been going on for months. I was dumbfounded. Not only would I never have believed Sophie was capable of this, I couldn’t understand how I hadn’t noticed anything was going on in her life – let alone something so momentous.I was hurt, confused and had an odd sense that I’d let her down. We both complained about our husbands but I assumed she was on the same page as me: it was just the normal marriage frustrations a couple of decades in.I had clearly underestimated how unhappy she was; how unseen and unappreciated by her husband she felt. Now, she’d been transformed from a dowdy, invisible mid-life mum to a minxy sex kitten. Her eyes sparked with excitement as she filled me in on the X-rated details: ‘I’ve been dying to tell you!’ she giggled. I hadn’t seen her so enthused in years. When my best friend of more than 30 years told me she was having an affair, I burst out laughing, assuming it was a joke, writes Eliza Fisher (picture posed by models) Sophie was sleeping with a man who wasn’t her husband; a guy she had met through work, whose attention and desire had invigorated her in ways she couldn’t have imagined before (picture posed by models)The reason that Sophie was finally confiding in me soon became clear. Her lover – also married – had initially insisted neither of them told a soul, but had now agreed they should confess to someone they trusted. They needed help – or, to be more precise, alibis so they could continue to meet.‘I’ve used every excuse in the book,’ Sophie laughed. She was exasperated by the effort of having to lie to the man she had vowed to be faithful to.She stared at me imploringly, safe in the certainty I would do anything for her, including this. Even if I didn’t approve, which, for the record, I didn’t. But she was my BFF – the Thelma to my Louise – we were ride or die for each other and always had been.And who was I to judge? I’d also made some questionable choices over the years.She’d held my hand when, before I met my husband, I repeatedly went back to an ex who was clearly bad news, never lecturing or pointing out that she had told me so. She supported me and listened to the same story over and over. This was what you did for a friend.Our situation was complicated by the fact that over the years of hanging out as families, our husbands were now friends. So if I told mine what was going on, he would have to lie to his mate, even just by omission. It felt too big an ask, and that the kindest thing I could possibly do for my man was keep him in the dark. It was for his own good, I told myself.I don’t think I realised then, while Sophie and I were making all these decisions about how it would work, that this meant I would be lying to my husband too – in a way suffering all the negative aspects of having an affair, with none of the sex or fun. I think I started feeling guilty then and there, in that first conversation, and carry that with me still.What worked to our advantage was the cliche that men don’t really talk. If our husbands met for a pint or messaged each other, they might chat about football or swap jokes but they wouldn’t discuss their relationships, or logistics. They were extremely unlikely to say, ‘Our wives are going out together this evening, aren’t they?’ Or, ‘Our wives saw a film together last night didn’t they?’Still, that didn’t stop me being a nervous wreck the first time Sophie told her husband that she and I were having dinner, but was instead meeting up with The Other Man in a hotel near her office. Eliza decided that the kindest thing she could do for her husband was to keep him in the dark about the situation as he was friends with her best friend's husband (picture posed by models)I leapt out my skin every time my husband or I got a notification on our phones. My husband noticed I was jumpy and asked if I was OK. On the spot, I came up with something about one of the side-effects of perimenopause being increased anxiety – shocking myself with how instantly the lie came out. I was ashamed, but relieved I hadn’t fallen at the first hurdle.As time went on, Sophie and I became complacent. If our families met up, I would put the affair out of my mind so I could still look her husband in the eye and laugh with him as normal. It was uncomfortable, but being a good friend wasn’t black and white, or always easy.The more we got away with fudging the truth, the less of a big deal it seemed. At the start, I would lie awake at night worrying, playing out in my head all the possible scenarios that could lead to detection, desperate to have escape routes – plans B, C and D, just in case.But as we got away with it time and time again, it seemed like I was panicking over nothing. It was, for a good six months, a complete breeze. By then, the only thing I was struggling with was not being honest with my husband.Even though I knew I had good reason – the secret I was keeping wasn’t mine to tell – this was the polar opposite to the way we usually operated. Our relationship wasn’t perfect, obviously, but the trust between us was the foundation of it all. Yet there were extenuating circumstances and I was sure he would understand if I explained, while praying that day would never come.In the end, a scruffy, little toy giraffe blew the lid off Sophie’s affair.About nine months in, she told her husband we were going to a spa hotel for the night. Earlier that week, my daughter had left Gracey Giraffe at Sophie’s house and her husband knew she was missing it, so decided to drop it round.I will never, ever forget the look on his face as I opened the door, no matter how much I wish I could. Or the way his expression changed, going from shocked, to confused, to the sudden heartbreaking realisation of what was really going on.I opened my mouth to make an excuse but luckily he interrupted me, because I had no idea what I was going to say. He held up a hand to silence me and said: ‘Please. Enough.’ He looked disgusted, appalled. They were the last words he ever spoke to me.Shaking as I closed the door, I ran to call Sophie and break the horrifying news. The call went straight to voicemail. I had visions of her and her lover enjoying a blissful hotel breakfast in bed in matching towelling dressing gowns after a night of passion, completely unaware that back at home, her life had just exploded.Later – much later – she told me she’d forgotten to pack her charger and had run out of battery, so she arrived back at her house with no idea of what she was walking into.My husband was furious with me for getting involved at all, never mind betraying him by being so consistently dishonest. He was incredulous that I was willing to put our marriage at risk to cover for Sophie’s affair.When he put it like that, I had to accept he had a point. It hadn’t seemed such a clear-cut decision in the moment. He insisted we see a couples counsellor as we try to rebuild the trust between us, as he felt like he didn’t know who I was any more.Thank goodness he could accept my fervent protestations that I only behaved this way because I was misguidedly trying to be loyal to my friend, otherwise I really think he might have ended our marriage.Sophie and her husband have stayed together. He ‘doesn’t believe’ in therapy so they’re trying to work through it on their own. She now thinks she was trying to get his attention with the affair and is committed to fixing the issues between them.Her husband no longer wants to see me or my husband. He thinks my husband must have known what was going on and that we were all laughing behind his back the whole time. He hasn’t banned Sophie from being friends with me, but if she says she’s going out with me, he finds it very triggering as this was the lie that facilitated her infidelity. Sophie and I hardly ever see each other now anyway, and when we do, it’s not the same. There’s so much simmering under the surface.I have a suspicion she blames me for answering the door that day and, if I’m honest, I resent the damage her affair did to my marriage. But we haven’t talked about it because we’ve had enough difficult conversations and rows with our husbands lately. We’re both exhausted.It’s hard not to notice the irony: I tried to be a good friend and, in the process, basically lost her. I can’t stop wondering how much worse it could possibly have been if I’d refused to give Sophie an alibi when she asked me. How could we be further apart than we are now?As this mess moves into the rearview mirror, I know I made the wrong decision. The problem is, I’m still not sure what the right one was.Eliza Fisher is a pseudonym. Names and identifying details have been changed.