As long as it's not a Touch or iPhone she should take a look at, well, any of the music management applications for Linux. If it is a Touch/iPhone then unfortunately iTunes is the only real way to sync.
Amarok is probably the most popular but is KDE based. If you are using Gnome it will require you to install the core KDE libraries which will take up a fair amount of room on her
SSD just to run a single application.
I prefer Banshee personally, I think it's faster than Amarok and a little easier to use. It's also native to Gnome which means she won't be adding a bunch of otherwise unnecessary libraries to make it run. It will manage playlists, music files, podcasts and videos.
Songbird is the latest and greatest to the scene. It's Mozilla based (like Firefox) and also does all of the above.
None of the Linux music management applications are store backed though, so she'll have to buy her music directly from wherever - Amazon, etc. or rip her own CD's and import the music manually which is just a few mouse clicks. If she buys her music through iTunes make sure she's getting the DRM free versions. None of the Linux based apps will play back DRM'd music to the best of my knowledge.