Lots of good specific advice here. Eating enough to start to gain weight is pretty important, especially pre stem cell transplant.
You mentioned tests taken to rule in/rule out possible kidney issues. Many folks with kidney complications find the induction therapy is sufficient to get the kidneys back into the normal range. Some do not. If that is the case for the two of you, sufficient fluids are important to keep the kidneys happy. Low protein, low sodium, low potassium and low phosphorous food choices are recommended, depending on stage of kidney disease. Potatoes, for example, are high in potassium. So are bananas. Dairy is considered high phosphorous, as are most nuts and nut products. Ask your medical team for advice on dietary issues, and/or consider adding a nephrologist to your team if the kidney involvement persists, particularly following the stem cell transplant 100 day measures.