Guy I'm seeing is "best friends" with a girl who is known to sleep around?

[wolfgang]

You're in a difficult situation here and atm as you're not official you have 2 options as I see it: 1) you ask him out - once you're together you can hopefully get him to cut down on seeing her, and he should be able to be committed or 2) you just play it cool, which he will find attractive as you're not clingy (once he compares the two of you, you should come out a clear winner) and in time he will get bored of her. Good luck