It is necessary to first understand Yoga physiology to answer this question.
Yoga physiology says that the human body and personality are contained in 5 sheaths or koshas:
Annamaya kosha, Pranamaya Kosha, Manomaya Kosha, Vignanamaya Kosha and Anandamaya Kosha.
Of these koshas, Annamaya Kosha is the gross body. The word anna means food and the name suggests that our physical body is sustained by food.
The next three koshas make up the subtle body.
The word prana means life and is the vital energy that permeates
all existence. Our physical life is sustained by prana. We die when this prana leaves the body.
This pranamaya sheath or kosha is more subtle than annamaya kosha but more gross than the other three sheaths.
The word manomaya means mental. This manomaya sheath is the mental sheath. This is where part of our mind resides. This is more subtle than annamaya and pranamaya koshas but more gross than the remaining two.
The word vijnanamaya means intellect and is the intellectual or discriminating sheath. It is the seat for rational thought and most people mistake it for the self.
The last Anandamaya Kosha is the causal body. The word ananda means bliss. This last sheath is blissful because it is very close to the blissful Atman. You may be wondering why this sheath is also called the causal sheath. The reason is that the record of all our karmas is kept in this sheath and since our karmas lead to rebirth this sheath may be thought of as causing rebirth.
At the center of these 5 koshas lie the Atman. The practice of Yoga in practical terms leads to digging though these 5 barriers and finally reaching the Atman.
A detailed description of these sheatths is in Taittiriya Upanishad II.1.1, II.2.1, II.3.1, II.4.1 and II.5.1.
Now that we understand Yoga physiology, we can answer your question. After death a person only has the subtle and causal bodies and the Atman. The Atman is always a witness whether in the earth plane or in hell and never suffers and is never a non-witness. It is the subtle body that suffers in hell.
I am adding the following reference that states explicitly the witness role of the Atman.
No act done with the aid of the organs of sensual perception, is ever
lost. The five sensual organs and the immortal soul which is the
sixth, remain its witnesses.
Mahabharata Anusasana Parva Section VII