Is it ok for my girlfriend to have dinner with her ex?

I am a and have been with my partner for 4 years. When we were in high school she dated a MALE exchange student for about 3 months. They have not spoken much since to my knowledge, although one time a few years ago I found out she was talking to him on facebook behind my back. He is coming to "visit" the states and wants to see her for dinner. She is making a special trip and missing work/school to have this dinner with him. I was very upset and crying hysterically, and she told me if I wanted to break up with her over it that is ok. I love her, and don't want to end our relationship, but I am sick everytime I think of this and it completely consumes my thoughts. Please help!