Strict diet or taking regular medicine?

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I attended my brother-in-law's wake and burial last week. He died of cardiac arrest. He had been diagnosed of this ailment since last year and was advised to take his regular medicine. However, my brother-in-law believes that he did not need to be dependent on the drug instead just manage his diet and do regular exercise. His discipline with regards to diet and exercise was indeed commendable. He remain to be the slender type of person I knew more that twenty years ago. What puzzle me is that, was he right in insisting not to follow doctor's order?