My grandmother had colon cancer so after all the surgeries and treatment she went through gas was just extremely painful for her. To try to avoid this she had to avoid all dairy, and most fresh veggies and fruits. I think the raw stuff was just too much -too much bacteria or something, but it caused painful gas. So if she eats a lot of salad, maybe have her switch to steamed veggies. Cabbage, broccoli, beans, fermented foods might be all things to avoid too. Red meat might also need to be cut down or cut back.
I also think it's a good idea to have her talk to her oncologist it's possible that worse bloating means the cancer is growing much faster than it should.
I would recommend she discuss dietary concerns with a Dietitian who deals with cancer patients.