Is my father's forgetting of the Quran after a stroke related to the strength of his faith, and are there any tips for him to regain what he has forgotten?

If your father's forgetting of the Qur'an was due to his negligence in reciting it, then that is blameworthy. However, if his forgetting was due to a compelling excuse such as illness or the like, then there is no sin upon him, for Allah does not burden a soul beyond its capacity. We ask Allah to make that an expiation for him and an elevation of his status.