What happened to your father is called edema. Gravity causes surplus fluid to gather in the feet. My husband even had it in his legs. The doctor prescribed a diuretic pill to get it rid of the excess fluid and he lost 20 pounds in 4 days! Of course, then one has to be careful of becoming dehydrated.
Test were prescribed for him during this time. He had an echocardiogram to make sure his heart was OK and ultrasound or CT or ? of his legs to make sure he does not have a blood clot. All was fine.
Next in my husband's case he is taking a medication for fast pulse rate that can cause edema plus he has not been active. Just in the past few days he is walking more and his ankles are no longer swollen.
Lastly, lung cancer by itself can cause some edema. So you can see there are many reasons for edema. What helped my husband was a diuretic, keeping his feet up and walking around more.
Hope this helps.
Trudie