It probably depends on what you consider junk food, but I think for a cyclist riding longer distances there is probably a lot more room for deviation even for a diabetic. As far as having to stop more often, why? It is possible to stuff a couple days worth of snacks in your jersey pockets and lots of reasonably healthy food keeps well enough to pack a couple days lunches if needed including fresh fruits and veggies.
I am not a diabetic, but I think that there is a lot of stuff that while maybe not health food is readily available and no so bad. I tend to have things like Jerky, fig newtons, fresh or dried fruits, nuts, and baked stuff in my jersey pocket. As a diabetic he may need to be more selective, but given the rate he will be burning fuel he will probably have a lot more latitude than normal. In a pannier I have lunch items like bagel pbj makings, hard boiled eggs, dried hummus, fresh fruits, fresh veggies (avacados, carrots, and cabbage kept well and worked for us), crackers, tortillas, foil packed tuna or salmon, hard cheeses, and canned chicken or other meat. Once in a while for a change of pace we bought a sandwich in the morning for later in the day.