If she is used to the Palm OS, I would not swap that on her. I had a TX for a few years and still like the OS. I lament its passing. I now use a Palm Treo 680 SmartPhone (with Palm OS) and I gave my TX to my son who still uses it.
The only reason I went over to the Treo was that it was a full-function Palm PDA as well as a full-function SmartPhone.
You might even find a new TX somewhere, on e-bay maybe; but, if you're in the market for one, I would recommend Squashman's as he's been around here a long time and is highly respected here.
If you do find a new one somewhere, be sure to ask if it comes with a full factory warranty. A lot of out-of-production new devices like that are what is known as "gray market" import and their warranties are good only in the country of origin.