Bfs female friend gives me weird vibes

So l've (22f) been dating this guy (26M) for a few months now. He has a female friend(22f) who used to like him. I only found this out after delicately asking around because I noticed how she was around him. He says he's only friends with her because she's dating his best friend:

To me, that's a bit weird. He says he's only friends with her because she's dating his friend. He's nice to her, sometimes too nice but I ignored it because they've known each other for a long time. However, I think she still has feelings for him. It's how she acts around him and I thought I was just jealous or overreacting until I saw how quick she changed up when she didn't know I was around.

She also bosses him around and uses his things like their hers. It took me a few days to understand how to explain how I feel about this to him because I didnt want to sound like l'm accusing him of anything and i dont want to yell at him. I don't think he would cheat on me, but her? I think she flirts with him and he doesn't even know. It's hard to explain but if you've ever seen/saw what I have when she's around him, you'd know what I mean. I didn't want to bring it up to him because I don't want him to suddenly feel self conscious about her and over think things. But even his guy friends thought she's a bit weird with the situation.

Surely someone else has been in a situation similar to this. I just want to know how it turned out. Should I trust this weird feeling? Or how should I go about dealing with it?

**TL;DR; ** : My bfs roommate/best friends gf might still like him. What should I do?