I'm so sorry about Bandit - you both are in my prayers.
When my girl was diagnosed and we decided to treat her with chemo, I started to read everything I could find and talked to some holistic vets. We put her on Essiac tea (not all are created equal - we got ours from a Chinese herb shop), colostrum because of the immunoglobulins which bolster the good part of the immune system and ester-C; I would mix up this concoction in a coffee grinder and give it to her mixed in cottage cheese. There was also a vitamin supplement we got from a holistic vet that I can't remember the name of but would happy to call the vet for if you're interested.
As far as diet, I believe very strongly this can make a difference - carbs are more easily available to the cancer than protein.
We were blessed to have her with us for 4 years after the diagnosis and, while I know that that truly was a blessing, I can't help but believe that diet and supplements contributed to that as well.