All other things being equal, a healthy diet is always better than an unhealthy one. But with cancer, not all things are equal. Cancer treatment can change your tastebuds, cause nausea, and affect appetite. When I was in chemo the only foods I could tolerate were cheese tomatoes and bread and crackers. I pretty much lived on tomato soup, grilled cheese sandwiches, and mac and cheese for months. Healthy diet? Of course not, but it beats no diet. Good food choices for cancer patients can be very different that for others.
Is he gaining a lot of weight? Is he a diabetic? If his current eating habits are not causing new additional health problems, then my suggestion is to leave him alone. There is nothing you can do to force him to change and nagging him to change when that may not even be in his best interests right now only increases stress for both him an you.
Everything we eat, whether it be refined sugar to a t bone steak is metabolize into sugar. That is the only way our bodies can utilized what we eat for energy. Therefore, sugar basically feeds our bodies and every cell in it, cancerous or not. Without it we die.