Tell her to either place all of her sexual energy in other activities e.g exercising, gardening however if she can't stop then it fine for her to masturbate in my own personal opinion, I'm an 18-year-old female who does it to, I mean everyone does in one form or another, don't place any blame on her cuz the guilt afterward is the worse and you don't want her to feel ashamed about it. A tbh is ****ing amazing once you orgasm so I would say try and distracted herself from doing and if not it's fine to want to do it