Re: Bad friends
Alright, IMO you should approach your gf as such: "Look...I keep getting the feeling that you and [best friend] have been flirting a lot...I know you're loyal to me, it just upsets me." If she's a good gf, she'll listen to your POV and not blow up. Of course, it's up to you to not accuse her like, "Will you quit flirting with my best friend?!?!?! GOSH!!" Yeah, bad.
You might also consider talking to your friend, as you're obviously close to him as well.