I have breast cancer and although it isn't the same, I don't eat any sugar or things that act like sugar (grains, natural sweeteners) if I need to sweeten something I choose stevia or erythritol as they don't have the same effect on blood glucose and therefore don't feed cancer cells.
Cancer requires glucose to grow, anything you eat that converts to sugar feeds the cancer. I eat under 10grams of fructose a day and often I don't even eat that much, a few berries here and there. I try not to have lactose so I don't drink milk and if in my coffee I use double thick cream, and I eat hard cheese but sparingly. It has also been proven that exercise drastically increases chances of survival from cancer, so if they are up to it they shouldn't stop exercising. I had surgery to remove the tumour but went against medical advice when they wanted me to have chemo, radiation and hormone therapy (yep all 3 I was supposed to have). For me I just couldn't subject my body to such nasty treatments that also caused cancer, for me the risk wasn't worth it and I researched and researched and the more I did the less I wanted treatment.