What's a healthy diet for a cancer patient?

Same as for others. A balanced diet rich in fresh vegetables, fruits, whole grains, nuts; and low in saturated fats should be good for a patient with cancer as it is for other people.
Veg, fruits, water. A diet very high in vegetables and some fruits. Raw fruits and vegetables are the very best, lightly steamed can be ok. Juicing of fruits and vegetables is also very good for you. Avoid saturated fats, avoid sugar, and carbs. Eat very lean meat sparingly, such as turkey, chicken, and fish. Tree nuts are good for snacks. Avoid soda and drink just plain water or green tea.