You need to explain to her that cancer cells are different from the rest of the cells in the body in that they can only run on sugar. If you go into ketosis, making your body run on ketones, the cancer cells have no food and die off. So, the take home message is every time you eat sugar or refined carbs that turn into sugar quickly, you are feeding your tumor.
I am NOT advocating firing the oncologist. Absolutely follow their advice on real medical treatment. Dietary therapy is an adjunct to that however that can help get any "stray" cancer cells out and not let them take hold anywhere else.
She is very lucky that they caught it early. My heart goes out to her. Take good care of her.