Do I let him remain "close friends" with my GF?

Me and my girlfriend attend different universities and do not see each other very often. She's become very close friends with this guy and this girl. She has told them that she has a BF and she talks to them about me often. A couple weeks ago she told me that her close guy friend that he sees everyday and are "close friends" slapped her ass and has been playful with her one day. He slapped her ass knowing that she has a boyfriend so it was intentional. I talked to her about this and she has told me that she wants to remain close friends with him because she can't make friends and she'll be lonely and miserable at school. I personally don't think its okay to let a guy that doesnt respect his friends relationship to remain "close friends", but what do you guys think? Am i overreacting? Do I stay and live with them being "close friends" or do I just leave?