Riboflavin or B2 deficiency, if treated early can help within days of treatment. It usually occurs in both feet. Just a little extra riboflavin is needed in the diet. Unfortunately Mareks can cause curled under toes and walking or sitting on the hocks as well.
There are several leg bone...
I would suspect possible Mareks disease if you have been feeding them a good balanced chicken feed. Have you tried giving riboflavin or some human B complex vitamins? If you lose her, I would get a necropsy and testing by your state poultry vet.