Hello there we understand your concern towards your fathers health. Having flat foot is not a condition and it is not a defect too. It only lacks the arches. But because of the flat foot there are some disadvantages such as knee pains and weight bearing will be a issue because if the loss of arches. Since it so genetical we cannot treat.it medically but surgically it us done be altering the structure but can't whether that will work. For now ask your father to place his feet in slightly hot water with pinch of salt added in it. It should ease.his problem to a certain extant.