What do you consider proof of a friend's spouse's infidelity?
This question is prompted by this question.
What do you consider enough proof to tell a friend that their spouse is cheating? Obviously, walking in on an irrefutable situation – passionate embrace, missing clothing, in the act, is one thing.
What else constitutes enough proof to “tell?” Do you confront the cheater first, so that they know you know, or do you blindside them? What if you misread the situation, and the guy with his arm around her is her brother or cousin?