His age and anything he did that might have caused minor or greater damage, particularly a strain, might be the explanation. Whatever has happened is unlikely to get any worse during the weekend so he should simply exercise caution regarding use of the leg and rest it as much as possible, including elevation. Having said that, I know of someone who spontaneously (no obvious cause) came down with an infection in the knee and, yes, put off going to the medical profession until he was basically immobile and, it is said, so swollen that his lower leg looked like he had filariasis. He was told had he delayed much longer he would have lost the leg.