Is this guy still courting this girl? If he's letting her hang out, stay there, etc. maybe she has some issues at home or something. Or he's trying to make the point he wants to take care of her. Is she truly milking you guys, or is she just chilling in his room? Does she want to stay there when you aren't there? Or do you say "I'm leaving, you need to go"??
My roommate has a girl he is dating too. She comes over and hangs out. She isn't living there, but I don't mind her being there as long as I am there.
Maybe just tell the guy how nice you are for letting the girl stay over so much and you are usually a nazi about these kind of things.