Is she a pretty sophisticated 17 year-old? Do you know what perfumes she does like?
This is just me, but I found Une Fleur de Cassie really difficult, and I still don't like it much. It's not scary and animalic like Le Parfum de Therese, but it's a bit skanky (I like animalic and skanky, by the way).
If you do want to go with Frederic Malle, I think Lipstick Rose, Lys Mediterranee, or En Passant might be more appropriate for a young girl. If you're open to other perfume brands, maybe let us know why you chose Cassie, and maybe we can help with other suggestions?