Your friend's father needs :
1) Good control of sugars with appropriate medications prescribed by his physician or diabetologist
2) Good review of foot by a surgeon with regular dressing till the wound heals
3) Appropriate diabetic footwear so that the wound in the foot doesn't extend. Also under such circumstances, there is excess pressure on the other foot so chances of injury
in the other foot increases. Appropriate footwear as prescribed by foot surgeon will help avoiding these kind of complications.