Hmm. A Wrinkle In Time
by Madeleine L'Engle? She was a Christian (albeit a rather peculiar one), and there are Christian themes in the book, which is a fantasy and a great, but less-well-known-than-it-deserves classic. The main character is in her teens. ...OK, wait, I forgot, the book does have magic in it... but it's worth looking up anyway. If the parents don't object to Narnia-type magic, they might not mind this?
A Summer To Die by Lois Lowry is very sad, but a great book. No magic, no teenage rebellion--it's about a girl whose older sister, who has always been the prettier one, dies of cancer. The family goes away for the summer to a cottage so the sister can die in happy surroundings, and the main character makes friends and develops her photography talent and assists at the birth of someone's baby, and stuff, all with the sister's illness as a shadow. It's hard to explain, but the book is very subtly written. She might end up bawling her eyes out, but it isn't trite or corny, and it was one of my favourite books at her age. Still is, actually!
The only other author I can think of is Elizabeth Goudge. They're adult books, but should be fine for her--eminently respectable, slightly historical. She writes with a wry, witty Jane Austenish air and has gorgeously quirky characters--A City of Bells
would be a good one to start with, maybe. A few of her books have references to fairies and Celtic mythology, but not in an offensive way. Towers in the Mist
is good if she's into Elizabethan stuff; or Gentian Hill
is good too. Think of her as Jane Austen crossed with Dickens and just a touch of Lucy Maud Montgomery, but less soppy, and there you have it.
ETA: Thirding the Limberlost
recommendation. It's uncommon enough she might not have read it, but a genuinely good book and a classic.