I just noticed i sometimes have the same problem. And really, the only thing to do is to try to think as much as possible about your position, at least for 1 week or so, until your body does it alone. It is much harder, because you have to concentrate both on the playing and on the position.
As far as specific advice, i would recomend her to keep the shoulder as low as it gets (because that is natural). Energy stucks in places where muscles are stiff. If it's getting serious, it might be good for you to go and press very lightly on her shoulders while she is playing. If she keeps on playing with a stiff shoulder, it will be very harmful and very hard to corect later. My teacher used to be very hard on me when my position wasn't correct, because she understood the implications.
Hope this helps.
Bitus.