The Nancy Drew Notebooks are a different series, for younger readers, and much easier than the actual Nancy Drew series, FWIW.
I think the Narnia series is significantly more challenging than what she has been reading, but if she finds the books compelling, she can probably get through them. Nothing improves your reading like stretching a little! And she can always ask for help.
I think Christmas gifts are a great time to give a book that is maybe a little beyond what they're reading now -- it can always wait in the bookcase for when she's ready, if it turns out to be too hard now, and those books also make great read-alouds.