I am surprised that David didn't mention this but today many new oral cancer patients, who never smoked or drank had HPV as a causitive factor, so your father is under a misconception about the cause of his cancer which is more likely caused years of smoking and excessive drinking.

In any case, the possibility of recurrence is much higher, whatever the original cause of the cancer, if smoking and/or drinking are continued post treatment.

Being a former alcoholic myself I can tell you that drinking 7 bottles of beer a day is alcoholic drinking.