Whoa...since she is a beginner, I would recommend 72 mm wheels. Most beginner skates have 72 mm wheels. Even the ones I use are 72 mm, and I'm an intermediate skater. 72mm wheels are also easiest (and cheapest) to find replacement wheels for. Basically the physics are the larger the wheel, the faster you go, and the less control you have. That is why trick skaters use 50 or 60 mm wheels. Since your gf is a beginner, she probably doesn't want to be going too fast to start.
Also, if she will be rollerblading a lot on concrete, wheels eventually wear out. Each wheel has its own hardness, usually ranging from 72A to 86A, with 86A being rock hard. As far as wheels go, the softer the wheel, the smoother the ride. For casual skating, I'd recommend 78A or 80A wheels. For hockey, I'd get wheels as hard as possible. Of course, you won't need to worry about buying wheels until your gf wears out the ones that came with her blades.