I just finished Splendors and Glooms, which might be right up her alley. It's set in Victorian England, about a girl who vanishes right after seeing a puppet theater at her birthday. It has magic/fantastic elements in it, but most of those are offstage, not the center of the novel. The novel itself revolves around Clara (the vanished girl), and the two poor children who work for the puppet theater. Definitely age-appropriate, definitely historical, and definitely complex use of language.
Another thing you might try is the bestsellers of 150+ years ago. You shouldn't have to worry about sex, although there may be romance, and the society can be complex and interesting, and of course totally believeable. I really enjoy Anthony Trollope - try Barchester Towers if she's not intimidated by big heavy books, or The Warden if she is.
