Is it ok for my girlfriend to have her male (best) friend as her roommate?

I've been going out with this girl for about 2 months and its been sort of a long distance relationship so far. I see her every 2-3 weeks and stay for about a week and a half each visit. I'm seriously considering moving to her city to be closer to her and go to college there. She lives with this guy who is her best friend and she claims its completely platonic between them. She's lived with him for about 3 months and has been friends with him for 3 years. She told me she liked him a long time ago (not sure exactly how long ago) but he never seemed to like her back even after giving him every opportunity to. The guy doesn't seem too fond of me even though I tried to be as friendly as I could. He's always in a bad mood and doesn't treat her (or me) with a lot of respect when I'm there. They dressed up and went out for dinner/drinks last week and thats when it really started to bother me since it sounded very similar to when her and I went out to dinner the last time I was there. She is also VERY considerate of him and feels bad he is left out when her and I go out to dinner or to a movie. The whole situation is bothering me a lot lately. Am I just overreacting or do I have good reason to be upset? What should I do?