What happens if an alzheimer's patient is also diagnosed with cancer? would either affect the other?

Usually Not: However, if there is cancer in the brain it could make symptoms of alzheimer's worse. Additionally, alzheimer's make make it difficult for a patient to understand the cancer they face and go through curative treatments that can be difficult on a person so it might make more sense to simply treat the symptoms of a cancer rather than try to cure it in a patient with advanced alzheimers.

There are not: Root cause relations, nor is treatment of one or the other. I have been awed by the decisions that spouses and family members make in such cases. We each may have preconceptions for ourselves, but not for us to judge. Our duty is to outline options and liabilities for patients with any co-morbidity.