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

[Ava]

wow, i really hate girls like her. sounds like she doesnt really want to share him with you, trying to stake her claims on him by posting that photo and flirting with him and claiming they are best friends and everything... and it really complicates things since you arent actually his girlfriend.. but if he's bestfriends with a girl like that then maybe you should think about avoiding that timebomb by forgetting about him altogether while you still can... because even if you start dating, "talking" to him about her isnt going to make her go away. shes always going to be a threat. unless hes going to get her to stay away, which i doubt.. its really up to you what you do though. are you willing to fight for him? would you be able to handle being with him if he wont give her up? girls like that never really go away unless the guy MAKES them. and its rare that happens.. and when it does, everyone will assume you're the bad guy. ugh.. it just gets ugly. if it were me, i would just stay away from him to avoid that whole mess. if you are important to him, he should ask you out.. and tell her to lay off next time she tries to flirt.. but in my experience, girls like that can never be trusted and guys usually fail to have the proper boundaries regarding them.
but yeah i guess if you really want this guy, you should ask him out. and talk to him about her, and hope for the best. even if you arent dating it probably wouldnt hurt to tell him how she makes you feel, since apparently you two already kind of have a thing going, right? if hes a good guy he will stop flirting with her and put your feelings first, and even tell her to leave him alone if she doesn't back off on her own.
its just, if you couldnt tell, im sort of bitter about this sort of situation from my own experiences.. but I really hope it works out well for you, whatever happens. communication will be needed, and boundaries. good luck.