The skin changes mentioned by you indicate a complication of diabetes. When blood supply to the feet is reduced due to blockage in the arteries supplying blood to lower extremities ( legs and feet), the skin turns dark and black with other changes like dryness, scaly skin. Alcohol worsens diabetes complications among which nerve damage and peripheral vascular disease are also present. The person must stop alcohol consumption as it can only make matters worse and should visit a Diabetes foot care specialist for treatment. Does she have symptoms of neuropathy also like numbness in feet, loss of sensation, pricking sensation and cold feet. You have not mentioned completely whether she is a diabetic with complications like heart and blood vessel disease, any other complications etc. She must take care of her feet and check them regularly for any cuts or ulcers that do not heal.