Help me stop a serial cheater

Here we go: I have been in a relationship with my g/f for two years. Anyways, two weeks ago, I was out with a bunch of my buddies for his birthday party. Most of my buddies are single, so unfortunately, going out to clubs to 'beat puss' is their every night mission. I just happened to tag along for the event. We went to an upscale club and had a VIP area reserved for him. As the night got deeper, our area was attracting women. There was a group of women, 8 or so all married, out for a girls night out. They joined into our party because quite frankly, my buddies that don't know what love is, could care less if they were married, single, old, whatever.. as long as they had the 'puss'.. that's all that matters. Now this one woman began to grab my arm and we all know why women grab arms. I immediately texted my g/f who began to flip out. I reassured her that I will leave if things progress this way. She starts talking to me.. I let her know I have a g/f.. to which she says well Im married so what.. I stopped there.. didn't tell my buddies I was leaving because they were with the other women and as I was leaving.. that woman smacked my ass and told me shed see me around.. I was like wtf? The next day, she somehow found me on facebook (thanks to my friends) and sent me a friend request and a message stating she liked me, i left to early, and thinks we should hang out to which I responded no that I have g/f. Told the girl, she flipped out, wants to cut the woman. About 4 days ago, I find out one of my buddies is hooking up with her and he is completely bragging about the 'b1tch' as he refers to her. He tells me that she still wants to somehow see me agian.. seriously? I do some investigating, her fb says she is married with two kids. I check the guy out. I feel so bad for this guy. He is just an innocent looking dude that loves his kids judging by his pictures. You can somewhat see stuff on his wall like his check in status places with the POS wife he has. My question is.. how do I expose this to the guy (my gf has my back when I do) without my buddy getting murdered by him? P.S. The woman is about mid 30's and my buddy and I are early 20's.

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