To add to what Dr. Drib wrote, just download MobileRead's Mobi Download Guide
. It's a "booK' with links to all the mobi formatted books on MobileRead. She just add the download guide to the "documents" folder of her Kindle then she can browse through the titles and when she sees one she wants, she clicks the link and it will automatically download to her Kindle (as long as the wireless is on).
Feedbooks has a similar Kindle Guide
. Between the two guides she'll have lots of books to choose from.