Is prayer obligatory for a senile father, and should he be prevented from smoking if he abstains from food and drink because of it?

If a father reaches the age of senility (dementia) and his mind becomes confused, then prayer is not obligatory upon him, and he will not be held accountable for any deficiency in it. However, if he regains lucidity during prayer time for a duration sufficient to perform it, then it becomes obligatory upon him, because the intellect is the basis of religious obligation (taklif). You must help him pray and gently and wisely prevent him from smoking.