First, while women will state men can be oblivious to their advances, women can be just as bad.
I was in a long distance relationship with my gf (now wife) for quite a while, though not as bad as yours. She had a TA who was obviously comming onto her (gave her extra points in class, invited her to some tree hugging group meetings, etc) and she just wouldn't believe that he was anything but a nice guy, after all he knew she had a boyfriend
. As a male you know how it works, but a lot of women tend to just miss the obvious, that being most guys are not just 'nice' to you if you are cute and of dating age without wanting to get in your pants. Well anyways University rule was if a TA dates a student they get kicked out so he never asked her out....until of course she was done with his class. It was a nice 'I told you so' moment for me.
Anyways if your girl is like that you have nothing to worry about. Sooner or later he will make his move (and he will if given half a chance) and she will tell you about it.
Then there is the chance she is the type that will cheat on you, and if so then think of this as a blessing, its much better to find that type out early rather than later.