How do I get my roommate to dump his sorry ass girlfriend

Ok-I live in an old farmhouse my parents own so its rent free. I have a roommate that pays rent. He has a crazy girlfriend that he has dated off and on for years. The other night we were partying and she got so drunk she pulled her pants down and peed on the floor in the living room. I was like that is gross you gotta clean it up and get her outta here. he said it is no grosser than my dogs peeing on the floor.(i have two pugs who aren't completely potty trained) So my girlfriend cleans it up and my roommate says lets take her home. So we drag her into the car and half way to her house she wakes up and attacks her boyfriend who is driving the car. He gets over to the side of the road, but is covered in scratches and his nose is bleeding. I basically jump over the back seat and hold her down while we continue drivng. the next day i tell him she ain't allowed in the house no more. He gets pissed and says he pays rent so he can have her over. how do I stop him. I need the rent so I can't kick him out.