Actually, Frenchbabyface, I DID have it happen, twice, but in opposite ways.
I broke up with my fiance. I didn't have a reason I could put into words, I just knew something wasn't right. A few months after the breakup, one of my so called best friends said my ex had been cheating on me. He knew about the cheating, but didn't tell me because he didn't want me to get angry. This is a person I had been tight with since freakin' 6th grade for God's sake!! I told him I would never forgive him for not telling me, and for having so little faith in our friendship.
A few years ago, I saw one of my girlfriend's man out at a club. He was with another woman. He tried to play it off, but I'm not stupid, and I know the difference between platonic friendship (which I have always had) and a date. I told my girlfriend the very next day. She wasn't angry in the least, she was glad I told her. She elected not to break up with him immediately (they were getting ready to go on a cruise, and she didn't want to ruin the trip), but they broke up soon after returning.
Like I said, if you are my friend you will know I am not jealous, I am not interfering, I am simply providing you with information I believe you should have. I would never tell my friend to leave a relationship, that's not my place. As a matter of fact, when I was dating my husband and we were having problems, one of my girlfriends told me to leave him. I elected not to, but she remains my friend to this day. Being able to speak openly and honestly is a requirement for me, and people whose friendships require lying and subterfuge aren't worth my time.