Girlfriend has a lot of guy friends and one crossed the line. What should I do?

Last night me and my girlfriend were hanging out and she was texting her "best friend" . He asked her what she was doing and she told him hanging out with me, the boyfriend, and told him i said hey. I have met the guy, seemed like a nice guy. He all of the sudden kinda flips on her and she doesn't understand why. Then he proceeds to tell her he is tired of hearing about me and tired of being the friend. that he is in love w/ her and wants to be with her. Well this blows her mind and she starts crying hysterically, because he crossed a line. She has been friends with this guy for five years and neither of us expected this. I know they care alot about each other. The thing is, she cheated early in the relationship and ive had problems with her being too flirty with guys. I dont mind the guy friends, but now its like i dont know who to trust, because they all seem to hit on her. I dont know how to handle this, because I am almost at the point where I am going to forbid the guy friends or dump her. I dont know if I can handle worrying about her every time she goes out with one of them. I dont want guys hitting on my girl. I am in love with her though and dont really wanna lose her. Any advice?