It's sad, but if he just doesn't see her that way, it can't be helped. Maybe someday he will, but... there are other ways for close relationships to develop besides romantically, which people tend to realize later in life when they start meeting more people and their previous friendships grow older.
Not to dismiss her feelings though. Romantic love to adolescents feels as real as romantic love without the hormones and inexperience, especially when they aren't aware of any other form of love. It still hurts, but she's tough and can move on