What your dad is experiencing is normal, after the death of a mate. You can make sure he eats, his clothes are clean, do a little housework. Take him out, to a movie or dinner. Don't avoid the topic of her death, if he wants to talk about her things she liked listen. If he has friends, encourage them to visit. He needs to start being active again.
Make sure he has a good support system..
Good Luck,
Angel