The answer to your question is going to depend on whether you want a good fit or a she'll be right fit, how much you are prepared to spend and how much effort you are prepared to put into the search.

I'm just shy of 5'2" myself and I've gone the route of a custom frame with 650c wheels and custom cranks to get a realy good fit that I realy enjoy riding. EDIT: OK I need to say that the custom frame is still under construction but I have set up an IDENTICAL riding position on the 24" MTN bike and its the best riding position I have ever had. I have a Giant XS bike with 700c wheels which I can ride but its not honestly that comfortable and unless I'm realy dedicated its easy to decide not to ride it.

XS frames with 700c wheels are a "she'll be right mate" type of proposition for a 5'2" person. Terry bikes would be good. 650c at least.

OK, a question for you. What sort of riding is your girlfriend going to do?

I've bought myself a 24" wheeled MTN bike and fitted it with slick road tyres and various other good quality components and honestly its a very good bike. A little heavy but very comfortable and honestly I can realy hammer on it.

One important issue to consider is crank length. See http://www.cranklength.info

Regards, Anthony