You can always ask your doctor for a diuretic and see if it works. There's no serious danger, but it may not do your father much good, and there are
Diuretics work well for most kinds of fluid retention because they cause the kidneys to excrete more fluid. But if the cause of the edema is lack of liver function, as appears to be the case with your father, then it's more complicated than other types of fluid retention. When the liver isn't functioning well, it isn't making adequate proteins. Proteins help balance body fluids by sucking fluid out of the tissues and back into the blood serum. In your father's case, what's needed is more protein to reduce the swelling in the tissues. Unfortunately, eating more protein won't help because when you eat animal protein, the body has to break it down into amino acids and then rebuild those into human protein -- and it's this protein-building process that isn't working.
A diuretic medication will remove fluid from the veins and arteries and cause it to be excreted by the kidneys, so it’s worth a try. The hope is that water will then be pulled from the tissues into the blood to rebalance the internal pressure. But the risk is that without adequate protein, this second step may not happen, causing vascular dehydration. This, in turn, causes dry mouth and other symptoms of dehydration. And since many cancer patients already suffer from dry mouth, this isn't a good side effect to add to the mix. So it's reasonable to try a diuretic medication when your father first starts to experience swelling, but it's a short-term fix at best. A better approach is to have him wear support hose (if he doesn't mind them) and elevate his legs as high as is comfortable to fight the effects of gravity. Massage may also be effective in helping rebalance fluids.
some potential side effects.