I am sorry for the loss of your father. A reasonable diet and proper exercise does not cure anything, especially cancer. It does help us to resist diseases for a longer time, in some cases long enough to survive treatment. Realistically, it may be luck of the draw. But it is also important to have regular screening as well. Even if someone decides for themselves that they will not undergo treatment.
The ironic thing with my Dad was that he was diligent about going to Dr.'s. And you'd think with the cancers he had before, they would have found the colon cancer, if it had been there as long as they say it had been. He had a sigmoidoscopy about 6 months before he was rushed to the hospital with a bowel obstruction caused by the advanced colon cancer. Oh well.