Is what we did by performing Hajj on behalf of our deceased father—who used to fast and perform Hajj, but was negligent in his prayers—sufficient to protect him from the torment of the Fire and the grave?

Neglecting prayer is a grave sin, and one who abandons it completely is considered an apostate, outside the fold of Islam, based on the Prophet's (ﷺ) saying: "The covenant between us and them is prayer; whoever abandons it has disbelieved." As for the one who is negligent, praying some prayers and missing others, he is not considered an apostate. It is permissible for his family to dedicate the reward of some of their good deeds to him and to frequently supplicate and seek forgiveness for him.