Hi Alison,
if you are taking any of the TKI's...imatinib, dasatinib or nilotinib you should not eat grapefruit or drink its juice. This is because it contains an enzyme which interacts negatively with the chemicals contained in this class of drugs.
As for a general diet for cancer patients to follow... I do not think there is one other than to say that you should try to eat a rich and wholesome diet with as few chemicals/additives as possible. Lots of fresh fruit and vegetables (preferably unsprayed with chemicals) and maybe decrease the amount of protein based foods. But that is a debatable point. Personally I try to follow a way of eating that separates different classes of foods... like protein from starch foods for instance. So for example I would not eat meat/fish/cheese/eggs together with potatoes/rice/pasta or bread and vice versa. I would instead combine vegetables/salads with starch (as list above) or vegetables/salads with protein. I try to eat fruit separately from any other foods.
Since I have followed this way of eating I have had little digestive upset and feel generally better. However, I am not sure this is exclusively for cancer patients.
Drinking plenty of water (apart from that in tea/coffee etc) is my mainstay and I find really makes me feel better.
Sandy