I would suggest a few different characteristics to consider:
1. Memory. How many songs would you like for her to be able to save on it? You're somewhat limited in that price range, but you could probably get a model that allows her to save a couple hundred songs. Personally, I think the more memory the better.(also, you can forget about repairing it - if it breaks it'll be cheaper to buy a new one.)
2. Display. Some models have a display on it so she can see what's playing; others do not. Which would she prefer?
3. Flash memory v. hard drive. If she has an active lifestyle, get flash memory because bouncing around won't impact it.
Thanks, I never even thought about display. One more thing to worry about. You mentioned the advantage of a flash memory over a hard drive, is there any advantage of a hard drive over a flash memory?