We had a problem like this with my suitemate and her boyfriend. Luckily, she wasn't my roommate. We talked to the girl about him being around constantly and being obnoxious and annoying, but she either didn't care or didn't take us seriously. He was in our suite all the time even though he wasn't a student. He made insulting remarks which he would laugh away as jokes. The only thing that stopped the guy from coming around was a measles outbreak that shut the campus down and only students and employees were allowed to enter.
The following year I wound up sharing a house with three woman. One decided her boyfriend also lived with us and stayed every single night. This is how the situation resolved itself. One day the mother of the problem housemate called and another housemate answered. Problem's mom asked her how everything was going. My housemate told her EVERYTHING! YOUR DAUGHTER AND HER BOYFRIEND LIVE HERE TOGETHER, AND WE ARE MISERABLE!
Problem and her boyfriend had gone away for the weekend and had locked the bedroom door which contained her answering machine which had recorded the entire conversation.
When Prob and her boyfriend returned after their weekend, she heard the machine and let out the loudest scream that I had ever heard and started crying. They moved out the next semester. Personally, I didn't think that it was right of her to tell Problem's mother this information, but it was effective. BTW they are married now with babies.
I think that you need to decide if you want to maintain a friendship with this roommate or not. You can handle it like an adult and tell him directly that you don't want his girlfriend in the room when he isn't there. When you have a roommate, you need to compromise and that seems fair to me. I think that that approach is the best.