My 82 year old mother-in-law has always taken care of herself but now she is dependent on outside care. She has congestive heart failure, renal failure, diabetes, just started dialysis and learned last week she must have a toe amputated. She says she doesn't want to live this way and would rather stop dialysis and die.
Seeing what she went through, I think it is sometimes the most merciful thing to do to stop the dialysis. I would stand behind your MIL in the decision she makes. She may change her mind day to day, because death is frightening. She will know when she is ready to move on to the other side. I am so sad that your family is going through this. I know how hard it is.