Don't buy a frame that you're unsure of especially when you're not sure what size will fit your GF. First thing you should do is at least an online fit calculator of some sort so you can know her dimensions and apply them to a frame. Better yet would be to test ride a couple sizes but if you're buying online that's gonna be difficult.

I will ask though are you sure she wants a bike or is this your idea of surprising her with something you're into? cause I've seen this story play out many times when a guy thinks it's cool to build up a bike and it just ends up parked against a wall and used as a clothes hanger. Just wondering, either way good luck.