I would. It just isn't in my nature to allow a friend of mine to hurt. Which is why most of my friends know that if they are cheating and I am friends with both them and their partner, to keep it from me. If I find out, I'll feel obligated to tell them, because I would want someone to do the same for me. As the lovely commenter above me pointed out, it can be a matter of health as well as the emotional aspects.
I would likely confront their partner first, depending on my relationship with them. My best friend's boyfriend and I get along well, if I caught him cheating I would tear him a new asshole and make sure he was honest with her. I'm protective of my friends but I don't feel it my place to be the bearer of the news unless it was a particularly complicated situation. He has a chance of redemption of he confesses, apologizes, and makes an effort to never do that again.
I would. I can't bear the thought of holding something like that inside and my friend whom I care a lot for not know. I've had similar things happen, where I knew something my friend should have really known. Not cheating but other things similar.
I definitely would NOT say a word. Been there, done that and it was the worst three years of my life. Yes, three years!! Now, if she had set me to task to find out if he was doing things behind her back, that might be a different story. But, if I just stumbled upon a situation and didn't know for a fact what was going on, I would stay out of it. I might get a little cheeky and go up to them and embarrass them and make them think I'll tell their wives if they don't.
The only people in charge of protecting or shattering the trust between the couple are the individuals in the relationship in question. It's not me. That, I know for sure.
Depending on the relationship, maybe. I would most likely confront the "cheater" first and talk to them about it. I think it should be their responsibility to come clean with their partner. I would like to think I would want to know if it were my partner - but if it were somebody that was not a close friend coming to tell me this, I would probably just be embarrassed and pissed off.