My nephew was also reading at a much older level when he was nine or tennish. He adored Agatha Christie, the Poirot novels. He and his grandmother used to read them aloud to each other, my nephew doing the accents. I seem to remember buying him an Anthony Horowitz novel around that time.
Does she like humour ?
The Polly and the Stupid Wolf books are a delight…by Catherine Storr.
And don’t forget the Just Williams !
Lynne Reid Banks ’ books for teenagers and children are excellent…a very good one is One More River about a young girl trying to adapt to life in Israel after her parents decide to emigrate from Canada.
I have always believed Silas Marner by George Eliot is a good intro to the classics for young readers. It is nice and short and such a heart warming story.
Happy browsing.