What to do when you see your friend's boyfriend with another girl

[SATX*APhi]

My mom has always told me that if I ever saw a friend of mine's boyfriend with another girl, I should never say anything to my friend because she'd always believe her guy over me. My mom says that most women won't want to believe their significant other is cheating, even if they saw it with their own two eyes, so they will go through denial and blow me off becaues of it.

I know this is a very touchy subject. I really don't know what I'd do if I saw my friend's boy all up on another girl, but wanted to see what you all would do. Would you comfront him when you saw him? Would you tell your friend? Would you talk to someone else about it first (someone that doesn't know either of them, obviously). I'm kind of, sort of, in this situation, but not exactly. It got me thinking about what my mom told me though. So what do you all think?