Do they need to be novels? The Lad is very enamoured of the -Ology books. They lure the reluctant reader in with lots of bits and pieces (envelopes to open, fold-out bits, embedded dragon skin, and so on), but there is a fair bit of reading there that they can pick up bit by bit as well. Wizardology and Dragonology would be the obvious place to start, but there are plenty of others.

Also, you could try the How To Train Your Dragon series. Fast paced and plenty of illustrations.

Do you know what it is about the Harry Potter series that she particularly likes? The wizarding, the boarding school setting, something else?