Catch her off guard and flirt back. Then when she actually thinks she has a chance...hint at her gf. She will stop. Unless she's and wants a ..then you're literally screwed!

She has a gf, but she hits on you? >.> Isn't that kind of shady behavior even if it was a dude? I would say "ignore her", but you guys are homies so... Just respectfully say something along the lines of: I'm not interested in women, and I'd like it if you stopped trying to come onto me because I feel like it is straining our relationship a bit? I dunno. ._.