My dad has been diagnosed with merkel cell carcinoma, he has had an lymphectomy and tumor removal, is there a better diet to do?

No special Diet. Merkel cell cnacers can be dangerous especially if it had spread to the lymph nodes. In terms of diet, eating healthy is good for all cancer patients. This means low fat diet which has a regular serving of fruits, veges and salads at each meal time (5-6 servings per day).
Yes. The merely cell center at the university of washington on seattle has one of the largest experiences and research labs devoted to merle's cell cancer in the world. They are finding that nutrition is intimately related to managing and controlling merle's cell cancer. While I don't know much about the specifics of the nutrition reccs I would recc contacting them and discussing reccs.