Some cancer cells have genes switched on for fructose transporters, so they can take up fructose from the bloodstream. Normally only the liver takes up fructose from the bloodstream to detox it.
Cancer cells slurp up fructose, US study finds | Reuters
As fructose is more damaging to DNA than glucose, with a greater risk of triggering further mutations in tumour cells, its best for him to avoid sucrose (which is 50% fructose) and fructose as much as possible. Plus fructose puts a strain on the liver.
A low carb diet high in medium-chain fatty acids, e.g. coconut oil, with plenty of fresh vegetables as the only source of carbs is probably the best diet, and will boost the immune system and liver function. Starch is certainly preferable to sugar.
F 5 ft 3. HW: 196 lbs. Primal SW (May 2011): 182 lbs (42% BF)... W June '12: 160 lbs (29% BF) (UK size 12, US size 8). GW: ~24% BF - have ditched the scales til I fit into a pair of UK size 10 bootcut jeans. Currently aligning towards 'The Perfect Health Diet' having swapped some fat for potatoes.