I might be able to lend you some insight into this. I'm 5'3" and just went through an exhaustive search for a new road bike to use for racing. In addition to that, I have a sister who is 5'2" who I just bought a bike for. It's not easy finding something that fits well!!! I ended up doing a very extensive bike fit session at a sports medicine center and then took the numbers around to various dealers to get an idea what might fit well.
I looked at bikes by Time, Titus, Cannondale, Specialized, Orbea, Calfee and a few others. Most of those are pretty pricey since I was looking for a lightweight bike to use for racing.
I got my sister (an entry level rider) a Specialized Dolce in a size 48cm. That's the size I ride. Prices are between $500 and $820 depending on whether you are buying an '06 or a closeout from last year. It's an 8 speed triple with a lower shimano component group, but it's not a bad bike for an entry level rider. One nice thing about the Specialized bikes is that they use 700cc wheels even in the small sizes. They also make a 44cm, but your GF might be too tall for that one. It really is a tiny bike. There are also a few levels of Dolce model depending on how much you want to spend on components.
Trek also makes a nice women's specific bike, but the ones I was looking at (Madone 5.2 and 5.something else) are in the several K range. However they may have an entry level model in an Al frame, you might want to check their website.
Most of the bikes I tried didn't have small sizes or women's specific frames. Many times the smallest I could find was a men's 51cm or 49cm. I felt pretty stretched out in those. You can fiddle around a lot with stem length and seat position, but then if you get too far off the proper size then your knees might not be in the proper position for the best power output.
I'd say do a fitting at a place that will put you on a serotta size cycle so you can get a set of measurements for an "optimal" bike. That will suggest proper stem and top tube lengths for a bike with a certain seat angle, etc. then test ride a bunch of things and see what she thinks feels best.
PM me if you need more info