I use my Palm as an mp3 player, and it works after installing an applescript for iTunes. It won't work as easily as with an iPod, but with a little initial setup, it should work fine.
First, you have to make sure that her mp3 player will mount as a drive on your computer. It should (I think). The way to tell is if it shows up on your desktop when plugged in to the computer. You should be able to eject it as you would any other CD/drive etc.
If it does, then you can download an applescript. Either check out Doug's Applescripts (there is one there that I've seen but never used) or look on VersionTracker for a script called iTuneMyWalkman. That's the one I use.
After you've installed the script, just follow the instructions to set it up. For ITMW, connect the mp3 player and mount the drive. Then, go to the script icon in iTunes, (it's in the menu bar), and click ITMW Preferences. Choose the appropriate drive and folder.
Then you have to set up a playlist in iTunes called ITMW. To make it easier, I use a smart playlist that contains all my 4- and 5-star rated songs, and is limited to 300mb sorted by rating. You can set it up to automatically beging the sync when it detects the drive (which might take a little while, so be patient), then to automatically unmount the drive once it's finished.
Good luck, I hope all works well for you!