BACK OFF!!!

Okay so, this girl has been flirting a lot with my boyfriend since her boyfriend moved away. He hangs out with her and she follows him around like a puppy. She is one of my really close friends. And I don't want to lose her because I think she's overstepping her boundaries. And I don't want to lose him because she's more attractive than me. Idk if I'm being jealous or over thinking it. Idk if I should talk to her. I talked to my boyfriend about how I feel and he said i shouldn't worry about her. But idk.