I'm in LA for a few days, but I was really interested to read The Frisky's take on whether or not you should tell a friend if you suspect that their partner has been cheating. I don't know if it's just my social circle, but my American and UK friends seem to be on completely opposite sides of this debate. My American friends all say they would want to know, while my UK pals almost universally wouldn't. They say that what happens between couples isn't any of my business, and that if the person getting the bad news chooses to believe the cheater, I could lose the friendship (this article calls this phenomenon 'shoot-the-messenger syndrome').  

This is all true (and I have lost a friendship because I told someone I saw her man kissing someone else, which he denied...) but I still think that I would tell someone if I knew that their partner was screwing around. Because, at the end of the day, I would want someone to tell me.