Does man know when he will die, and does he know his destiny and what happens to the people of this world, and does he know if he is among the people of Paradise or Hell, and what is your advice to one who prays yet denies the existence of God?

It appears that these questions stem from whisperings (waswas) that should not be indulged. The deceased knows that they have departed the worldly life and transitioned to the life of the Barzakh upon witnessing the angels of death and the departure of their soul. They know what awaits them, whether it be bliss or torment, and they see their abode in either Paradise or Hell. As for the deceased knowing the fate of the people of this world, there is no evidence for that. The default is that creation does not know anything of the knowledge of the unseen except what Allah wills. Whoever prays while truly denying the existence of God is a disbeliever, and their prayer is invalid. It is advised for anyone who denies the existence of God or doubts it to reflect upon themselves and what is around them, for the entire universe bears witness to the existence of Allah the Almighty.