Will the deceased son—who was over twenty-six—be held accountable for not performing the prayer, knowing that he was mentally handicapped and not considered to be morally accountable?

The insane and the mentally deficient are not obligated to perform prayers or other religious duties because they lack intellect. Therefore, if a woman's son is insane or mentally deficient, he will not be held accountable for neglecting prayers. Mental deficiency (al-'atah) is a defect in the intellect that causes disorder, making its possessor speak confusedly. The mentally deficient person is relieved of religious obligations according to jurists, just as Allah Almighty is more merciful to His servants.