Hello. Thank you for coming to JustAnswer with your question, and welcome.
The doctor is talking about a deficiency in protein that is common in the elderly with chronic diseases, mainly because they don't eat enough. There are other causes as well, other common causes are kidney problems, and heart failure. If the protein in the blood is too low, fluids tend to shift into the spaces between cells in the rest of the body. So you will see generalized edema, meaning the swelling is all over her body.
This is hard to alleviate. The usual treatment is to try to correct her nutritional deficiency in protein and give her diuretics. However, keep in mind that a body, especially one that is very old such as your mother-in-law can only handle so much medication, or diuretic. The medication can very easily cause side effects, or may not even work if her kidneys are not functioning well. Medical management of this type of edema depends on the underlying cause.
Her recent IV fluid therapy may have also aggravated the condition.
For you, the major concern would be to get her to eat enough protein and calories to boost her albumin (protein) levels. But, here again, specific dietary needs also depend on the underlying condition causing the protein deficit. If her doctor is saying that the problem is only because her protein is low (and not because she has impaired kidneys), then you can simply attempt to get her to eat more protein foods. Calories will also help the body utilize the protein where it is needed, rather than using it for energy. Also, she would need plenty of fluids to go along with the diet.
Having had many, many elderly patients in my lifetime, I can tell you that this may be much more difficult than it sounds. So, I wish you and your mother-in-law all the best.