Aaah, zooropa ... what a pickle!
If you do act on your feelings you run the very real risk of;
Losing your girlfriend
Ruining your friendship
Ruining thier friendship
and then when you and your new, NEW girlfriend (possibly) split up ... you're left with nothing!
If you're not happy with the girl you're with, do the decent thing and let her down gently ... but for your sake I wouldn't plough straight into one of her friends, it'll just make you look like a scumbag!
Oh, and have a look at Aesop's Fable -
The Dog and the Bone!
J