In the case of uncontrolled diabetes, there are express manifestations in the legs also. Usually the 'extremities', -those parts which are far away from the heart the circulatory system get early and easily affected. Thus the legs and leg nails are easy targets. That is what happened in your father's case also.
While we get to observe e our hand often and know its changes immediately, it is not so in the case of legs. Many times we ignore our legs. We do not give proper care and hygiene to our legs even in normal cases. In the case of diabetics it becomes imperative and all the more important to take care of legs.
It is because of this that 'Diabetic foot care' is given priority these days.
In your father's case as he is already under a doctor's care, let him sincerely and seriously follow the doctor's advice. The first priority is to keep the sugar level under control. Let your father co-operate to make that aim fruitful.
Your father's nail infection will get cured now with proper treatment., but let him take all care for preventing any similar recurrence . Sugar level control and preventive foot care are the only remedies. AVOID all self medication.