We don't know what his lifestyle is.
It's likely there is only a slight chance a diet could help with your husbands case and that could depend on what his current lifestyle is.
If he is overweight, drinks to excess, eats a lot of fat or sugar and takes no exercise then it is likely changes will improve his health which could help if his cancer tries to come back and it might be something no-one will ever be able to prove.
I eat a fairly healthy diet and exercise a lot although it's not easy to resist those temptations. I have cut back a lot on alcohol and cheese and milk. Although I hope I haven't still got it.
If you're the cook I'd suggest gradual change if it's needed and perhaps more walking as a start.