He is denying the torment of the grave on the grounds that if he were to open a grave he would not see any change
Shaykh Muhammad ibn ‘Uthaymeen (may Allaah have mercy on him) said: One who denies the torment of the grave on the grounds that if he were to open a grave he would not see any change may be answered on several points, including the following:
The torment of the grave is proven according to sharee’ah. Allaah says concerning the people of Pharaoh (interpretation of the meaning):
“The Fire, they are exposed to it, morning and afternoon. And on the Day when the Hour will be established (it will be said to the angels): ‘Cause Fir‘awn’s (Pharaoh) people to enter the severest torment!’”
And the Prophet (peace and blessings of Allaah be upon him) said: “Were it not that you would not bury one another, I would have asked Allaah to make you hear the torment of the grave that I can hear.” Then he turned to us and said: “Seek refuge with Allaah from the torment of the Fire,” and they said, “We seek refuge with Allaah from the torment of the Fire.” He said, “Seek refuge with Allaah from the torment of the grave,” and they said, “We seek refuge with Allaah from the torment of the grave.” Narrated by Muslim, 2867.
And the Prophet (peace and blessings of Allaah be upon him) said concerning the believer: “His grave will be made spacious for him, as far as the eye can see.” Narrated by al-Bukhaari, 1374; Muslim, 2870.
And there are other texts too.
So it is not permissible to object to or reject these texts because of our mistaken notions, rather we must believe them and submit.
The torment of the grave happens to the soul, and it is not something that has a physical effect. If it had a physical effect it would not be the matter of belief in the unseen and believing in it would not serve any purpose. But it is one of the matters of the unseen, and the way things are in al-Barzakh cannot be compared to the way things are in this world.
Torment and blessing, and whether the grave is widened or constricted, are matters that the deceased feels but others do not. A man may see in his dream, whilst he is lying on his bed, that he is standing and coming and going, hitting or being hit. He may see himself in a confined and frightening place, or in a spacious and pleasant place, but those around him cannot see or feel that.
What we are obliged to do regarding such matters is to say “We hear and obey, and we believe and affirm.”