What if your best friend is trying to steal your boyfriend and he does nothing about it?!

Okay she is technically my ex best friend but anyway she is always all over my boyfriend. She acts like I don't even exist. But he doesn't even say anything to her. He just tells me everytime its not my fault----well do something about it___ but yeah HELP!? What do I say to him so that he knows I want him to change.!?
What if your best friend is trying to steal your boyfriend and he does nothing about it?!
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