I don't like the relationship my girlfriend has with her gay best friend.
Why don't you like their relationship?
I think he's violent and directs that violence towards my girlfriend.
It doesn't matter who, violence is never the right solution. This is abuse and you should contact the authority and have this reported so you can protect your girlfriend. She might be too scared to speak up.
I'm not sure why she lets him treat her like this, I'm going to confront him about it.
Talk to her friend and tell him that he needs to stay away from your girlfriend or you will call the police. It's not acceptable to be violent towards your girlfriend and it sounds like this man cannot be trusted.