Will a person who died while suffering from a mental illness that prevented him from affirming Islam be tormented?

Allah made the religion of Muhammad (peace be upon him) the final religion, and whoever seeks a religion other than Islam, it will not be accepted from him, and in the Hereafter he will be among the losers, as Allah Almighty said in Surah Aal-Imran/85. The Prophet (peace be upon him) also emphasized that anyone among the Jews and Christians who hears of him and dies without believing in what he was sent with will be among the inhabitants of the Fire.

As for the ruling concerning your brother who suffers from schizophrenia, if the illness does not affect his intellect and does not lift accountability from him, then he must embrace Islam. However, if the illness completely impairs his intellect and understanding: 1. If the illness was temporary and occurred after he had reached maturity and was of sound mind, then he is not excused, because he was addressed and accountable before his illness. 2. If the illness has been with him from the beginning or before puberty and continued with him, and he dies while afflicted with it before recovery, then he was not accountable or responsible for his actions. Allah will test him on the Day of Resurrection, in accordance with the Almighty's saying: (And We never punish until We have sent a Messenger) Al-Isra/15. On the Day of Judgment, four will present their arguments: a deaf person, an imbecile, a very old person, and one who died during an interval (fatra). They will be told to enter the Fire. Whoever enters it will find it cool and safe, and whoever does not enter it will be dragged into it.