Trust. Trust. TRUST. That is the basis of every relationship, and without it you hardly have anything to work on.
I'm sure it is a little threatening to see another guy liking your girlfriend, but you have to realize that she is with you for a reason. If your girlfriend had a thing with this guy she would probably be with him and not you. Don't get down on her because someone else thinks she is as great as you think she is. Think of it as a compliment.
With that being said, there does arise some issues with her hanging out with him all the time. Now, you can't tell her who she can and cannot be friends with, but you can try to put her into your position so she will be a little more caring when she is making decisions. You should talk to her about how it makes you feel when she is hanging out with someone that is obsessed with her. Explain to her that it would probably make her feel a bit weird if you were hanging around a girl that had a liking for you. While you are talking to her about this it is very important that you aren't accusational. Don't treat her as if she has already cheated on you, and definitely don't treat her like she has a plan to cheat. Doing that will only push her closer to him and farther from you.
All in all, just try to keep your head up and realize that if she wanted him she would have him. She loves you and that accompanied with a little bit of trust is all you need to make this relationship suceed. Good luck, hun.