She doesn't have much of a right to do what she did, ignoring you and what not, perhaps she was just really excited that her movie was going to shown on the big screen
She has two sides doesn't she? Surely you could have sat on the other side of her, said hi to the guy (as much as you hate him, be tolerant, this is sure to get some stars in your book). Replace the jealousy with trust, if she breaks the trust then don't go there again. Jealousy will only tear a relationship apart, trust me
Anyways, I hope the talk went well, sit her down, tell her how you feel about the guy, ask her how she feels about him, if she says he's just a friend tell her that you'll accept that, tell her how you felt at the movie place and that you weren't greeted by her seeing that she is your girlfriend and all.
Hope it all works out for you and her