16 GB should be enough if she doesn't put lots of movies/tv shows/music videos on there. In terms of music, that should be enough to have easily 200+ hours worth of music (may be off a bit, it'll depend on sound quality and such), which should be enough if one has a sense of control and don't try to put every single half decent piece of music on it.
Video, on the other hand, takes up a ton of space. On my IPod, I have about 7 seasons or so worth of TV shows as well as some movies and they take up like 55 Gigs.
If she doesn't put too much video on it, the only other concern would be all the apps that are available. There are some really nice ones that are free so most people have a ton of apps on their touch, but they may take up alot of space (I don't know since I dont have one myself)
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