I'll first note my pro-Glock bias, with a personal favorite being my Glock 26. I think it may be the appropriate gun for your GF, primarily because of the fact that she's proficient with the 17. Real self-defense situations are high-stress and the last thing you'd want her to be thinking is "Now, how is this different from my Glock?" With a very, very experienced shooter this may not be a consideration. For most of us, it's critical. When problem-solving abilities go out the window under stress, muscle memory takes over.
As for the concerns about carrying a Glock in a purse, I'll just reiterate what others have said--it's never a good idea to carry a gun loose in one's pocket or purse. It's unsafe and increases the likelihood that the gun will be out of position when you need it most.
Now, if she's not as concerned about the whole concept of identical operation to the gun she's already familiar with, I'd say that I rather fancy the Kahr PM-9 or CW-9 as very well-made, good-shooting pistols that are light and compact. Overall, though, I think the G26 is the right answer for her, given what you've told us.
