I'm 5'7" with a 31.5" inseam. I've got long arms and a flexible lower back. I ride a 54cm road bike with a 54.8cm effective top tube and a 110mm stem. I can make a 52cm fit (with a 53something TT), but I prefer a 54. A guy I know who is about 5'10" rides the same bike with almost the same setup, only his seat is a lot higher.

I think that based on all our stats you can get a good starting point for her, but she's going to need to sit on a bunch to figure out what size is best. Different brands have different geometry, so maybe she'd like a 52 in one but a 49 or 50 in another.