What to do about a best friend and somewhat of a ?

true, the question seems mean and straightforward, but i want to know. my best friend is hanging around with a girl who sleeps around with many other guys. i feel like this girl is a bad influence on my friend and i don't feel comfortable around her myself. my friend knows that i feel that way, but her excuse is "well, when you get to know her she is so funny!" idk, maybe that's true, but when someone like her is around my friend all the time, i worry that my friend might go and do something stupid. what do i do?