If the wound is more than a centimeter size then, yes, there is absolute need to drain the pus as otherwise for a person with diabetes
it poses a big risk of spreading infection to other part of the limb. Please make sure his sugars are under control and then only he should undergo the surgery. Also a big coverage of antibiotics will be given to him and he should complete the course of those without fail. So in short, it is not only safe but it is also must for your father to undergo surgical drainage
of pus. Take care.