The term diabetic coma refers to a loss of consciousness, usually from a low blood sugar. The brain requires a constant source of glucose as its primary energy source. Without it, the brain malfunctions. When the glucose level is restored, consciousness usually resumes. When the blood sugars are low, patients can also have seizures or other neurologic problems or damage.
Diabetic coma can also occur when the blood sugars go very high.
The high blood sugars cause the brain to loose its water content and it can also malfunction when this occurs. The problems of very low and very high blood sugars can occur in patients who have type 1 or type 2 diabetes. I would think that the health care team taking care of your uncle would have been able to normalize the blood sugars within 24 hours. Therefore, I would ask them whether they think something else happened to his brain during the acute period of loss of consciousness that would explain his period of prolonged coma. When kidneys fail, you have to perform dialysis or kidney transplantation.