I know a dude that pulls chicks left and right and generally does not gaf. He's made out with half the girls I know and gets the lion's share.

He hit on my girlfriend one day while drunk, albeit when he didn't know we were together.

Today they run into each other and my girlfriend suggests they study together. I tell her my concerns, and then she goes on to try to tell me "no, you're thinking of somebody else. He didn't try to hit on me." I specifically witnessed this shit occur.

This isn't sitting in my stomach well. What do I do about this?

This is college, in case you were wondering.