Acute renal failure
it is good that the nephrologist thinks it is acute renal failure because that is more reversible. Tough as it is to be patient and wait, that really sounds like good advice at this time. Nature/God is a far better healer for the kidneys. Dialysis is tough and although it does the work of the kidneys for them, it often slows healing. Kidneys are smart. Give them a little chance. One of the hardest things in medicine to learn is that sometimes it is better not to just do something because you can, but to only do it if is truly for the best.
From having taken care of lots of people with acute renal failure, the nephrologists advice sounds like good advice.
It is not clear to me when she got the morphine, but it seems like it was after the surgery not during the surgery–so the surgery records would not have shown a low blood pressure, but rather the time just before she was moved to the ICU.
One other tiny suggestion. Since your mom cannot move her legs herself, perhaps physical therapy can help with passive range of motion. Everyone has to think about the whole person, but it really is true that after knee replacement, people do better if the knee is “used” so that it keeps its flexibility and strength is not lost. Passive range of motion is where the therapist does all the work but there is movement of the limbs to help them heal.