A girl is obsessed with my boyfriend, advice?

I have been going out with this guy for a couple of months, and he has been friends with this girl longer than he had even known me. I don't mind him talking to girls, I'm alright with that, but she acts like she is going out with him. She refers to him as her own and tell him how much she wants to sleep with him. I know that he isn't like that towards her, so what should I do?
A girl is obsessed with my boyfriend, advice?
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