What is the best nutrition advice to feed my wife who weighs 80 pounds?

Take heart. I believe that the author Joan Didion has weighed 75 for more than a decade and is moving into her late 80s. She relates in a film her nephew did about her that the neighbors call him complaining about her weight all the time, and she has never weighed a whole lot more than she does now.
Ask your wife's doctor to order a nutrition consult, give him the weight and tell him of your worries. Then, with the doctors records, and knowing the patient, you can try to help this a bit. My partner has decades of GERD and honestly has come to dislike food in general. He is quite thing. I look at him and think "Well, 80 and pretty spry so you must be doing something right!"

Protein shakes was recommended by my mother's nutritionist. My mother lost some weight on a pureed diet and lost about ten pounds, nutritionist recommended protein shakes Cabot Whey Powder, I mix it in her Cream of Wheat, add banana, milk and blend in the morning, and she loves it. She also recommended 70 gm of protein per day, I puree fish and salmon, mashed potatoes and a vegetable. I give my mom regular puddings with whip cream, little cheese cakes with pureed strawberries. I add the protein powder to her smoothies. She regained most of the weight she lost. Is it possible to consult with a nutritionist, via telephone or a telehealth call? I found the consult to be a tremendous help. Got lots of ideas, and recipes. Worth checking it out. You will need a doctors order so her insurance pays for the consult.