If she doesn't have any medical problems other than her hip, I would recommend a formula that has 355 calories per eight ounces. There are several on the market that would probably be available in your local pharmacy. As a supplement, choose one that is high in calories , average in protein , lactose free and good tasting. They come in many flavors as well. Fourth, you mentioned that your wife couldn't walk very far.
Minimal exercise does help to stimulate an appetite.
Encourage your wife to walk the same distance, only more often each day. It will build up her tolerance and endurance. I would suggest she see a physical therapist for exercises that would benefit her walking. One last suggestion, remember, it is more enjoyable to eat with someone else than alone. Your mealtime with your wife should provide companionship and conversation for you both.