I'm not sure the technical name for the format she's up to--about 80-120 pages, slightly wider than a standard mass market paperback, an illustration every chapter or so. The series we're just finishing, Beast Quest, says 7-10 years on the back, as does the one before that, Underworlds (Tony Abbott).
Natural disasters, secondary-world fantasy, and portal fantasies are the last three sub-genres she's read, I think, and mystery is okay too. And the female characters have to be the protagonists, not the sidekicks, especially not the sidekicks who keep needing to get rescued.
She has a Franny K. Stein book from her teacher, so if she likes that there's more of those. Oh, and she liked the first Creepella Von Cacklefur book, so I'll get more of those.
Ideally the prose would also be non-awful, but I made it through the Underworlds series, so I can make it through some similar non-grammatical and clunky stuff to get her girl action heroes.
(Note age/format limits, please; i.e., don't recommend Tamora Pierce. And if you're going to say "she's the sidekick but she's cool," please don't.)