The patient in question did not lose his toe because he had fasted during Ramadan. There is a danger in fasting for diabetics. But it has nothing to do with gangrenous feet and the lopping off of toes. If you fast, have diabetes, and you are taking either insulin or oral hypoglycemics you run the risk of hypoglycemia, or low blood sugars. Profound hypoglycemia can lead to a comatose state, even death if left untreated. The neuropathic and vasculopathic complications of diabetes are the accumulative result of years and years of hyperglycemia. In the context of Mr Dawkins' tweet, this is an important distinction. During Ramadan, a devout diabetic who fasts increases his risk of hypoglycemic coma, seizures, sudden death. What demonstrably won't happen is that his toes will suddenly become necrotic and require an emergency amputation.
This is pure dissembling exploitation. This is donning the false mantle of Scientist Sage strictly for the purpose of scoring points in an ideological crusade. He is not looking to illuminate or unveil hidden knowledge. It is choosing ideology over empiricism. Dawkins has fallen prey to the very logical fallacy that he accuses his theistic foes of committing everyday. He would rather bend reality or willfully misunderstand facts if the empirical results do not support a pre-determined conclusion.