It's very common for young dancers to over-rotate their feet in first position. The first suggestion I have is to simply not turn out so far. The dancer should be able to stand in first position with equal weight between the pads of the big toe, little toe and heel. Otherwise she will create a twisting motion at the knees and ankles, which will leave her vulnerable to injuries.
Pronation is not a quick fix, and awareness is key. These exercises will help her feel when she's pronating and self-correct.
She needs to become aware of whether she's gripping the floor with her toes. The role of the toes in first position is to help balance your movement, not to lift your arch or act as the glue that keeps your feet turned out. Sometimes it's useful to have young students take class in their socks rather than soft slippers, so they can feel the contact with the floor more easily.
Have her pay attention to whether she's standing evenly or pronating outside of class. If she chronically pronates, orthotics might be useful while building strength and correcting inefficient alignment patterns. Rolling her foot on a pinkie ball or massaging her own feet will wake up the muscles.
Place a light scarf on the floor and ask her to first scrunch the scarf, then pick it up and drop it in a container.
If she's anywhere near the beach, have her walk on the sand. It's a wonderful exercise that works the muscles of the lower leg and feet, since they constantly strive to keep us balanced. Walking over a line of pillows can do the same thing. A bigger challenge, and one that helps correct alignment patterns, is to stand on one leg on the pillow and balance for up to one minute.