Girlfriend flirting with guy on facebook?

My girlfriend and I have an awesome relationship and are very in love. She has this guy friend that she has had since she was in school. She has always insisted that they were just really good friends and I have had my suspicions but have taken her word on that. Today she left her facebook open and I decided out of curiousity to peek. I know that was wrong but I couldnt resist. She talks to this guy constantly while she is at work and they also flirt with each other. He has a girlfriend and is supposed to be marrying her. Right now I dont know what to do. The flirting isnt really bad, but just seems to be playful flirting on her side and "I wanna see your "" really bad on his side. I know what I feel like doing but I dont know Im confused right now...mostly hurt I think and let down...