BTW my favourite book at 14 was probably Riders! Plenty of adult content. I'm sure she has seen and heard it all by 14! I'd be more concerned that she was able to understand the book, and apply the concepts to her own situation.
I enjoyed shyboy by monty when i was 12-13 and its what got me doing parelli at 14. Also enjoyes the mark rashid ones and tao of equus