My boyfriend's best friend is a girl?

So as the title states, his best friend is a girl. She's a good friend of mine and really nice and we're pretty close. I don't want to, but every time he hangs out with her (which is like everyday) I get jealous. Maybe it's because she used to like him and just recently got out of a relationship, but I feel like they're always together. Every time we go somewhere, it's to her house, or wherever she's at. I don't mind it much, but it does bother me. And every time we talk/text, she always comes up in a conversation. He's said that he doesn't like her when she confessed to him a few months back, but now I feel he may start to. What do I do? ): I don't like feeling jealous.