Most likely, he switched from a junk food diet (one of the main causes of cancer) to a cleaner (less or no junk food) diet.

If he ate only veggies, then that is a form of fasting, which is good for beating disease, but not for long-term health maintenance. Such a diet will cause a loss of lean body mass, like it did with your uncle, so it is not a good long-term strategy. But if a person is facing death, it is better than continuing a bad diet.

I suspect that pure fasting, with intermittent eating of clean animal foods periodically to maintain lean body mass, would be far superior for beating cancer and all other illnesses, but nobody is doing any studies on that (yet).