This is such an interesting question, because it demonstrates how flexible and adaptable our bodies are! It turns out that, with respect the the kidneys, the body actually has a considerable reserve of necessary function, and one kidney has way more capacity than is needed to perform all vital functions. In short, having two kidneys is redundant and provides the body with extra security and back up in cause something goes wrong. When one kidney stops working or is removed surgically, what happens is that the other kidney begins to enlarge and its capacity to filter blood goes up dramatically.
Therefore, your father should be just fine with only one kidney. Obviously, there are some precautions that need to be taken. For example, people with only one kidney should engage in dangerous contact sports because damage to the one remaining kidney could be a major problem. Similarly, people with only one kidney should be careful to avoid any medications that could potentially damage the kidney for the same reason. Potentially kidney-damaging medicines include common medications like ibuprofen, so your father will want to talk about this carefully with his kidney doctor
or his primary care doctor
to make sure he takes the necessary precautions.