My boyfriend has a girl best friend?

To be brief about it, my boyfriend and I have been dating for about 6 months now and he has a girl best friend who currently is living in a different state. I usually wouldn't have an issue with this type of situation because I even have many friends who are guys. Yet, my boyfriend used to have a sexual relationship with her a while back or what most people look at as "friends with benefits." He had wanted more of a relationship with her at one point, but she wanted nothing more than sex. A few months ago he went to attend her recital in the state she lives in, and he stayed over her dorm with her and a few other friends for the weekend. I didn't go, but I also tried not to let my anxiety fly out the roof. Now, she's trying to set up a day when she can come visit him so they have dinner together and catch up. I told him that I wasn't necessarily praising the idea and he told me I had nothing to worry about. I asked if I could go to the dinner to introduce myself and just see who this girl even is and he said, "Not this time. That would be weird I think, but you'll get to meet her sooner or later...." Is this anything I should be concerned about? I ponder if she even knows he has a girlfriend.