Is it safe to NOT consume any kind of sugar for a long period of time?

Like for year? You see my father in law believes because his Dr. told him that ALL sugar is bad. So he hasn't consumed sugar in months. Not even a grapfruit he thinks is ok sugar to consume. He thinks now that they are all unhealthy. My sister gave me an explanation opposing the Dr. views on no sugar consumption. And she also told me how sailors use to get sick when they would go on their long expeditions if their fruit and veggies rotted because it brought forth a desease to not have sugars or soemthing like that. So anyone who can be insightful by answering my question please do so. I'll appreciate it.