I dislike my boyfriend's female best friend
Why don't you like her?
I have an issue with her because she is a slut
I see how you feel, but you should try to accept her because she is your boyfriend's best friend. Your boyfriend will not be happy if you try to say who he can be friends with.
I just don't like him being around her.
This is just your jealousy and insecurity talking. If you have no reason to think he's cheating on you with her, then drop it.