This is cheap, never runs out of battery power, and really quite easy to use:
Seriously though, there are so many different PDAs out there and you haven't told us what she's actually going to do with it. Does she just need basic PIM functions (address book, calendar, todo list)? Does she need to read and write Word documents? Excel spreadsheets? Does she need to be able to use a wireless network? Use email?
Once you have a list of functionality that is needed you can start looking at online reviews and what's available on some place like eBay.