my answer is a question...WHAT? so your female best friend is being, what, flirty? with your male boyfriend? i don't care about the gender, just tryin to get it right. if this is correct then what you do is, 1 of 3 things
1: you sit them down, individually and talk one on one with her first. remember he's just a boy. he probably thinks 'what?' let her know that you love her but a little to close for comfort w/my man.
2. go to him and explain to him that, again, it bothers you and if either of these people don't care or don't think it's a big deal, then with friends like her, who needs enemys, right? and he will act oblivious, duh.
3. and then there is this. what the dirty little french like to call menage 3. how the hell do you spell 3 in french? anyways girl.
i would seriously sit them down if it's bugging you that much. i mean for you to write about it, im assuming its not just simple flirting. so it's hard to give advise with limited knowledge.
just hang in there. "if you gotta ask....." (it usually means not good)
good luck there sister.