Relationship Help

My boyfriend and I have known each other for 6 months. However, there was this girl he met four years ago and they've been friends ever since. He had a crush on her, but she had a boyfriend, so they never went further than friends. One year ago, she moved. They continued their relationship through emails. I didn't like it, but figured she was far away, so she wasn't a threat. However, the other day, he told me she called and said she was coming to visit and that she wanted him to take her out to dinner for her birthday. Alone. He said yes.

She knows he has a girlfriend but she doesn't want to meet me and definitely doesn't want me going to her birthday. So, I told him that I don't want him to go. It sounds fishy! We're all the way in the East Coast and she's going to come all the way from the West Coast to spend her birthday with him instead of with her own boyfriend? And she doesn't want me there? And she doesn't want any of their other friends there, either? What makes him so special?

I expressed my concerns and he thinks I'm overreacting. He says it's not a date and that it's just two friends going out to eat and that she is still with her boyfriend and that they're very serious.

What do you think?