I don't want my creepy roommate to ask out my friends?

My roommate is a difficult guy. I think he's got a personality disorder. He's not the greatest looking guy either. In the past he has asked out one of my friends. They rejected him. When I have friends over he likes it because he doesn't have many of his own friends. Anyway, I have this friend and she is lovely- kind, beautiful and intelligent. My roommates and a few others are going to see a movie - of course we asked the difficult roommate if he wanted to come. He wasn't keen until he found out my friend was going. My friend is not interested in my roommate, she is interested in someone else. I don't want my roommate to hit on my friend, or ask her out. It would make it weird when she comes to my house. What can I do?