Best friend becomes my enemy?

No, definitely not! You've given her way too many chances, and she blows it all again within a few months. Don't ignore her, because then she'll just try to ruin you socially, as she always does, but just slowly distance yourself from her. Ask your friends to help, and ask your guy friend to stop talking to her. She'll never change, and she's always gonna be a whore. Sorry, good luck!