Do You Tell Your Friend That Their Sweetie is Cheating?

Last week when the Texas Rangers came to town to play the Detroit Tigers, former Tiger Prince Fielder's name came up on the air; I mentioned that he seems to be playing a lot better for Texas than he did for Detroit. Joey said that Fielder was probably distracted from baseball by his wife ALLEGEDLY cheating with one of his teammates and no one telling him about it.

Joey and I agreed on the air that you don't tell a friend when their partner is cheating because it will wreck your friendship. What do you think of that theory? Would you tell someone when you believe their significant other is stepping out on them? And how much evidence would you need before you were sure that there was a cheating heart in action?