For whatever it's worth... both myself and wife are cancer survivors. We are both cancer free past the most likely period of recurrence. Basically the best outcome one can hope for in these situations.

My advice to your friend and you: find things to take back control. The helplessness and lack of control as a patient are the tip of a downward spiral that is hard to escape.

To that end I recommend these two books that have helped me personally:

http://www.amazon.com/Foods-Fight-Cancer-Essential-prevent/d...

http://www.amazon.com/Anticancer-A-New-Way-Life/dp/067002164...

Oh and ignore the stats. Mine were very good, my wife's were really poor. Cancer is a very individualistic disease and battle. No two cases are exactly alike. We are both alive, don't let him lose hope.

The research I've read seemed conclusive that what you eat is as important or more important as how much. Intuitively it makes sense that giving you body the correct building blocks would make it healthier and that's definitely not just a function of quantity. And from what I've read the science supports it as well. Here's a relevant book about it:

http://www.amazon.com/Anticancer-New-Way-Life-Edition/dp/067...

While this book is not specifically about vegetarian diets, it does contain good information about diet and the effect on preventing cancer, along with other info: http://amzn.to/mg3jg

I liked this book which is less controversial http://www.amazon.com/Anticancer-New-Life-David-Servan-Schre...

Found the audiobook online one day dotdotdot