Hello. There is a simple explanation for good urine output even though body requires dialysis
support. Kidneys have two major functions: 1st is purification of blood with maintenance of acid base electrolyte balance and 2nd is removal of excess water from body. In lots of advanced kidney diseases (more than 90% function of kidney - lost), purification of blood is affected badly but water excretion is preserved due to residual functioning of kidneys. Dialysis is needed when either of them hampered. Best wishes for your fathers health.