Old Roommate makin a move on the girl I like, WHAT DO I DO?

Ok so I met this girl this summer while working at camp & she is the most amazing women I have ever met. Ever since the second month of working at camp i realised that I like her. & then my roommate from school came to work at camp & he started making moves on her, & you know it didnt bug me at first but me & the girl have almost known eachother for a year & talk all the time & I really really like her, but my old roommate is still making moves on her on like myspace & in person & it is really bugging me now. I dont want to say anything to him because well he is one of my best buddies but at the same time I am thinking that he needs to back the **** off. How do I get my old roomie to back off without being terratorial & what do I do if he doesnt?