All her books will still be available. She will just need to re-download if it didn't happen automatically. Amazon keeps a backup of everything. Well technically, it is actually more like they keep track of what you own. It would make not sense for them to backup the same book thousands of times for each individual users.

Anything else she added will still need to be manually added again of course, but at least the books have been made simple. Alleviates the fear that if you drop your reader in a puddle that you lose your library.

With great power comes Awesome irresponsibility.