Reddit mentions of The Language of Flowers: A Novel

Jane Austen, definitely. The language is accessible to a modern reader, and the moral themes of defining acceptable behavior and finding love are very relevant to a teenage girl. I'd start with Sense & Sensibility and then move on to Pride & Prejudice.

American classics every American girl should read: Gone with the Wind, Little Women, My Antonia.

For modern stuff, maybe The Hunger Games (it's violent, but if she doesn't mind that it's a fun read), The Language of Flowers by Vanessa Diffenbaugh (I'm reading right now - excellent!), The Rules of Civility by Amor Towles, Water For Elephants by Sara Gruen.