I'm not in a relationship, and never have been, so take my advice with a grain of salt.
The way I see it, you're all stuck in a relationship, controlled by one girl. That is never a good thing, and this is especially worrying because she's creating tension between you and your best friend. Controlling partners are a no-no.
I don't see this ending well unless you step up to the plate and do something. Nobody likes it, except for the girl, so harsh feelings will keep building between you and your friend. In a situation where two out of three people are very uneasy about the current state of affairs, something will happen. And I find it hard to see you coming out on top, even if you did get the girl. Your friend would hate you.
What is it worth to you to keep this guy as a friend? And what is it worth to you to keep this girlfriend? If you can't answer those questions, then you're asking the wrong people. If you want to keep your friend, bail out. If you want the girl that badly, confront your friend. Has to end somehow, and the way I see it, there will be more tears if you wait longer.
And, CloggedDonkey? If you take the majority of this thread's advice and flee, please remember that in our own strange, internety way, we're here for you. Not like an avatar over a computer could replace a real human face, but remember it anyway. [sub]I care, dammit![/sub]