There are many reasons for dry, ashy feet. Simple dry skin is one of them and should respond to a good moisturizer such as Eucerin or Bag Balm. Another common cause of scaly skin on the feet is fungus, commonly called athlete's foot.
If you have been using a good moisturizer on your mother's feet and the dryness does not seem to improve, the problem could be athlete's foot. You might want to try an over the counter antifungal such as Lamisil or Lotrimin. Your podiatrist can correctly diagnose the problem and prescribe a prescription antifungal if needed.