No husband should come between a woman and her best friend.

As I have learnt to my cost, there is never a correct way to tell your friend that her husband is being unfaithful.

At a drinks party at their house, I’d picked up the frisson between her husband Tim and a neighbour. The flirtatious twiddling with her hair, the way she held his gaze a little too long, the touching of his sleeve and the way she laughed at his jokes just a little too enthusiastically set alarm bells ringing. Yet my friend Sarah was oblivious. She described the woman, a divorcee, as a friend with whom she and Tim shared evenings out.

Of course, I didn’t say anything about my suspicions. Then one evening I’d been to the pub with a colleague when I spotted Tim kissing this woman in his car. He didn’t see me, and so I scurried away to worry about what to do next.

I tried to imagine what I’d want if the circumstances were reversed. Trust and honesty are extremely important to me and I’d like to think my friends wouldn’t stand by and let me be duped and ridiculed if I ever found myself with someone unfaithful. Yet the other side of the argument was equally persuasive: it was their business, their marriage. Why should I interfere?

What swayed me was imagining how I’d feel if I found out Sarah had withheld information from me. I would not consider her a true friend. I decided I had no choice: I had to tell her the truth.

“I need to tell you something,” I said to Sarah, my voice cracking. “I don’t know how to say it…”

“Say it,” she demanded.

“I think Tim has cheated.”

She let out a yelp like a wounded animal before bursting into tears.

I hated being the messenger of her misery and kept saying: “I wish it wasn’t me who had to tell you.”

We talked for hours and she drained every last detail out of me: where the car was parked, the time of evening, what she was wearing, what he was wearing.

Sarah called me the next day. She had confronted Tim and he had admitted everything.

Apparently, it had started as a drunken flirtation, but “got out of hand” and they had been having an affair for a few months. He blamed work stress and said it was “only sex”. He said he still loved Sarah and they agreed to work on their marriage.

I respected her decision, even though I struggled to understand it, and for the next few weeks my friendship with Sarah seemed to continue as normal.

We met several times and talked on the phone as well as exchanging e-mails and texts. Then one morning, a month after I had told her about Tim, I received an e-mail from her. It said simply: “I’m sorry, Sonia, but we can’t be friends any more. Tim and I are making a go of our marriage. It isn’t easy obviously and, unfortunately, having you around is a permanent reminder of what happened.”

She was dumping me. I was shocked and very hurt. What would cause her to do this? Was she angry that I had been honest with her? Had I committed an enormous social faux pas by reporting on Tim?

Baffled, I spoke to various friends – without revealing Sarah’s name – and discovered this extreme reaction is common. Apparently, if you inform someone their partner is having an affair, you can become the scapegoat: all the pain, resentment and heartbreak gets transferred onto you.

I didn’t want to lose my friend, so I rang her. Despite my pleas that I had her best interests at heart, she was adamant. “If you hadn’t told me it would have fizzled out and I would have been none the wiser. As it is, you have created massive problems in our marriage by interfering. Please don’t contact me again.”

I was left confused and hurt. I struggled to understand how she had “kept” Tim, her betrayer, while chucking me.

Counsellors say this is a survival technique when people are trying to save their marriage.

If you’re in a happy marriage with most of your needs met and your partner cheated, would you want to know the details? Many don’t want to know. For people like Sarah, it feels simpler and easier to live in the lie of infidelity. Normal, blinkered life can then resume.

It’s been a year since Sarah ended our friendship. Mutual acquaintances tell me she and Tim are still married and that life continues normally – ostensibly at least.

I expect in time Sarah will want to renew her friendship with me, either after Tim strays again or when she sees him for the weak-willed, untrustworthy snake he is. And I will forgive her – of course, I will.

However, this whole experience has convinced me that I will think very, very hard about revealing an affair should such a situation arise again.

When it comes to affairs of the heart, honesty might not always be the best policy.