I was still reading and enjoying Nancy Drew and Hardy Boys at that age. Children's books of course but very enjoyable still.
Artemis Fowl if she hasn't read them. The H.I.V.E series is also good. CHERUB is also great BUT the themes mature quite fast (class A drugs in the second book, maximum security prison and shivs in the third) so do make sure you're happy content wise. I think I also consumed young bond and Alex rider at that age.
I also consumed all of Conan Doyle (which did take a good while even at my pace!) and Agatha Christie at that age.
Not the best quality literature but I also enjoyed the Daisy Dalrymple books and there's about 20 of them to devour. Victorian Mysteries by Robin Paige I think might also be worth a look.
Harry and the Wrinklies (really good fun) and the sequel both definitely worth a read.
The mysterious Benedict society might also be worth a look.
Oh and a quartet of books about mythical beasts, I can't remember the name, will go digging. Edit: companion's quartet, Julia Golding.
Has she read the Percy Jackson books? Rick Riordan has a variety of other series as well, all excellent.
Not all of those will necessarily stretch her, and she may well have been capable of reading them for a couple of years now but if she's anything like me she'll want a range of books, many of which are simply a leisure activity rather than needing educational purpose.