Just have her try on a bunch at REI (with weights!) and see which she feels most comfortable with. The weight should sit high on her hips.
Personally, I've had this backpack for 7 years: https://www.rei.com/product/127756/deut ... ack-womens
Its gotten a ton of heavy use, its held up great, and is always comfortable, even on long trips. I love love love this backpack.