I'm sorry to hear your grandpa has cancer.
Has he chosen to follow a vegetarian diet, or is it something he has been persuaded or pressured into? If he doesn't like his diet, it will reduce his quality of life and possibly make him depressed.
What is actually known about diet and pancreatic cancer is that a diet *high* in meat and fat *may* increase the risk, whilst eating a lot of fruit and vegetables may reduce this risk.
There is actually no need for him to cut out meat completely if he doesn't want to.
I'm a vegan, but I'm speaking here as someone who's had cancer; a restrictive diet, particularly where there is no proven need for it, often reduces a cancer patient's quality of life and makes them feel guilty if they can't (or even don't want to) keep to it.
I know whereof I speak. After cancer surgery I was persuaded (NOT by doctors) to try a version of the restrictive (even for a vegan) Gerson diet. There were nights when I went to bed weeping with hunger because I couldn't face the bland and repetitive food I was 'allowed' I lasted less than two weeks and felt so much better when I started eating what the hell I wanted.
Having cancer and cancer treatment is hell on earth; unless his medical doctors have told him to follow a vegetarian diet (and if they have, I would ask for an explanation), then there is no reason why he can't include meat in a diet rich in vegetables and fruit.
I will probably get heavily thumbed down for this, I usually do when I reply to questions about cancer in this section. But though I'm a vegan, I'm more concerned with your grandpa's comfort and well being during his illness and his treatment than in seeing a reluctant and unnecessary convert to veg*nism.
My best wishes to your grandfather