bro, putting myself in your shoes, I would explode. I am mad from just reading your post. I went through a similar situation a few years back, my best friend's (she is a girl) boyfriend grabbed my girl one night at a club and asked her to go fuck him. I wasnt there and my girl didnt tell me for days because she knew my temper. Anyway when she told me I didnt act mad, but inside I was in a rage. So the next saturday, we went to the club, scum bag was there, my girl made me promise not to cause trouble. I saw him, pushed the girl he was hitting on out of the way and grabbed his drink and split his head open with it. I was in a rage. I even threw a bouncer off me when he grabbed me. I went to the bathroom and picked the glass from my hand and six bouncers came in and asked to leave or the cops would be called.
Anyway I would ask your chick for the no bullshit version, straight and honest what really happened and then go talk to the worm. Scare the piss out of him but be careful with the violence. I almost went to jail for that inncident.