Good luck to you and your daughter -- surgery can be intense!
I like the idea of eating & resistance training for your daughter to gain weight. A little more lean mass can make the difference between making it and well... not making it.... in certain cases such as accidents and in cancer. Eating provides the means for the body to repair itself and grow, and resistance training can provide the reason for the body to grow. Both are important and work well together.
The main idea of
protein powders is food. People often forget it, but the most it can do is provide food with a certain digestibility profile. This is where studies that the fitness community can help your daughter. Milk is mostly
casein which is a milk
protein that will digest slowly and help
prevent wasting.
Protein powder is usually
whey protein which is good for promoting anabolism. Combining the two (such as putting milk & whey together) would provide a mix that is similar to human milk (which is close to 50%, 50%).
Your daughter will most likely have the most success with eating a calorie ratio close to 35% of her total intake in
protein, 35% in
carbohydrates, and 30% in fats. I do suggest peanut butter as it is a good source of fats and some
fish oil in tablets. Swithing to whole milk will also increase your dietary fat intake. Try eating complete "meals" (containing proteins, fats, and
carbohydrates) seven (or more) times a day with each meal being at least 300 kcals. 300kcals is not much as a glass of milk is easily 80-90kcals and a pb&j sandwich will get quite a lot. Try increasing the frequency of the "meals" up to 7 a day and then slowly increase the amount of food that she will eat. Seven is also not such a big number considering there is 1) breakfast, 2) snack between breakfast and lunch, 3) lunch, 4) snack after lunch, 5) dinner, 6) snack after dinner, and 7)
protein before sleeping.
There is also a lot of foods that are "unexpected" sources of
protein. Animals and animal products usually contain good amounts of
protein. However, combining beans and rice will also provide a complete
protein that is suitable for human consumption. As for the meal before sleeping, you may or may not want
carbohydrates in them as they carbohydrate intake before sleeping may interfere with natual body function. Peanut butter and milk sounds like a good option if this is of concern for you.
As a source of food, I am fond of combining leftovers with egg whites to make a white omlete. Egg whites themselves contain no cholesterol, no fat, and no
carbohydrates. The nice thing about this combination is that you can put it in a reusable plastic container and eat it as a snack inbetween classes. It can be made into a full meal combined with some rice or bread. I buy egg-whites by the gallon. It only takes 3 minutes in the microwave to make a complete meal.
You both may benefit a consultation with a nutritionist as he/she can help you design a plan to help your daugther. Good luck to both of you.