If she likes to read, the most important thing is that the book is well-written. That is what serious readers care about.
I’m going to assume that if she’s a reader, she is a Reader, complete with a good vocabulary and a love of good writing. I’d suggest The Screwtape Letters and The Great Divorce, by C.S. Lewis, particularly Screwtape. That book is not just spiritually beneficial; it is a lot of fun to read. If she likes mysteries, the Father Brown mysteries by GK Chesterton are very good. Has she read the Rings trilogy? Those are by JRR Tolkien, another well-respected Catholic writer, although maybe something she’d think trivial, because of the recent movie treatments.
She might like The Diary of a Country Priest or The Heroic Face of Innocence: Three Stories, by Bernanos, also very well-written, but I am getting into books that I wouldn’t lay on just any 15 year old. (Likewise Brideshead Revisited, although I have not read that one–I mean the book, though, and not the movie.)
Although I have not read it, I have heard very good things about “Essays On Woman,” by Edith Stein (St. Teresa Benedicta of the Cross). She is a Polish Jew who became an atheist and then became a Catholic Carmelite nun. She was one of the martyrs of the Nazi concentration camps. Pope John Paul II considered her writings to an example of what he referred to as feminine genius. That might be something that a typical reader won’t be ready to tackle at 15 years old, however, as she is a philosophical force to reckon with.
If nothing else, it will be hard for anyone who has read the Catholic authors I’ve listed to conclude that Catholics are misled fools.