All great advice above. What fits one shooter doesn't necessarily fit another. I strongly recommend renting a bunch of different guns until you find one that fits your girlfriend and one she can manipulate and shoot well.One other piece of advice from personal experience. Do not assume that because she has small hands she needs the smallest grip size. I was working with a female friend who decided on the M&P 9. She bought one and came to a basic tactical pistol class with the small grip on the pistol. She was having a hard time keeping her strong hand high on the grip which made it more difficult for her to control the recoil. During a break, we switched her to the medium grip and it ended up being much more comfortable for her and because of the shape of the grip, actually pushed her hand higher on the grip and into the beavertail giving her more control.
Best part of the process is you both will get to fondle and shoot a bunch of guns! Have fun!