Um.

Tell your friend. She never said not to tell 'em. Also, does he know she cheated?

It was the dumb kid who fucked a chick he knew had a boy friend, he should think before he acts. You can't always get away with that shit, so definitely tell your boy so he can show him what happens when you do that shit.

Its not the girl your protecting here, so I don't see the dilemma. Your protecting some douche who shoulda known there was potential for an ass-kicking for what he did.

You say the chick fucked up, but the guy knew she had a boyfriend? No doubt, she fucked up, but you can't kick a girls ass so might as well let the guy take the fall.