Your friend's affair

2. Getting involved wrecks your friendship. What starts as confessional sessions over drinks can escalate out of control. Lucie, 34, had no qualms about helping out her best friend, Helen: 'Just after Christmas, she arrived, all tearful, with her clothes stuffed into two carrier bags. Her marriage to Nick - also a great friend of mine - was over.' It soon transpired that Helen had omitted to mention that she had been having an affair with a work colleague, the real reason for leaving her husband. 'That changed everything. I was furious with Helen. I couldn't have her living here, seeing this other guy. So I had to ask her to leave.' Not surprisingly, their friendship hasn't recovered.

3. Her affair can sour your own relationship. When my friend Samantha's affair was at its most complex, my partner became moody and distant. Quite reasonably, he felt threatened by two women, constantly huddled together, discussing extra-marital affairs. Your friend's love life can even make you question your own relationship: is it really so perfect? And if she can get away with an affair, could you?

4. You don't have time to play counsellor. Being an unofficial therapist gobbles up enormous quantities of time and energy. Perpetually cancelling drinks with your friend is the coward's way out: 'Saying, "We must meet up" - and having no intention to do so - is counter productive,' says Professor Palmer. 'Make a firm arrangement, or forget it altogether. It feels more empowering than all that guilt.'

5. No one needs 3 a.m. phone calls. Your friend is getting double the sex; all you're getting is a red-hot ear from lengthy phone conversations - and sleep deprivation.

6. You are not her only friend. Don't listen to protests of, 'You're the only person I can talk to about this.' Would you load so much responsibility onto someone you cared about? Ask her to stop involving you. Never step in as an alibi (if you let something slip she'll blame you and forget all the times you've saved her bacon). If necessary, get firm and tell her that knowing every intimate detail is like being involved in a menage a trois - without the sex. And remind her that this is her affair. Not yours.

Do you have a difficult friend? Have you ever been involved in a friend's affair? Talk about your relationships with like-minded iVillagers.