It really depends on the friendship, but most likely, I would, even if it meant sacrificing the friendship.

I would bring it up. Her health and emotional well being could be compromised from a cheating spouse.

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.

yeah i probably would

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 want to be told if it were me, so yes, I'd probably let a friend know if they were being cheated on.

I'd probably talk to her girlfriend first, and tell her she needs to confess.

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 know I'd want to be told, too.

i would because i know i would want to know

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.

It's impossible to say what I'd do, as I've never been in the situation.