Tricky Situation Part 2

Here's another one.....

What if you know that a friend's BF/GF is cheating on them? Do you tell them? Or do you keep quiet because, again, it's not really your place?
This is a tricky situation that I actually have some experience with. I knew a good friend's BF was cheating on her when she was away on study abroad. She was a really, really good friend of mine, and there was no doubt in my mind that I should tell her. I would have wanted her to tell me. That was my reasoning anyway. Well, I did tell her. She was, of course, upset. She confronted him. He denied it, and they ended up staying together. While it didn't ruin our friendship, it was certainly strained after that, and honestly, not really ever the same. Even years later, I've run into the BF, and after a few beers, he will still bring it up and insist to me that it was innocent. Yeah right. I totally walked in on him and this girl. Where were you trying to put it? Her purse?!
Still, a really tricky situation. If a really close friend knew that Tim was cheating on me, I would definitely want her to tell me. If it was more of an aquaintance, I'm not sure I would believe it. And what about when it's the other way around? What if YOU are doing the cheating, and someone tells your partner? Well, of course that sucks cause if you're cheating, you're probably hoping your partner isn't going to find out. Ouch.
Oh, cheating, what a big fat f**king mess you make of everything.