[If a person’s mind is lost due to illness, how is his estate to be treated?]

Answer: If the matter is as described in the question, the wealth remains in the hands of the heirs of the sick person, because they have the best claim to it, and leaving it in the hands of someone merely to keep it may lead to its loss. The heirs may continue working in agriculture or trade as the sick person used to do, and they are entitled to receive from that what they deserve for their work.
They may give the sick person’s wife – or wives – what they are entitled to of maintenance, clothing, and what follows from that, out of the sick person’s estate. If there is a relative of the sick person whose maintenance is obligatory upon him, they should also pay that relative’s maintenance in a customary manner. Beyond that, nothing is permissible for them; rather, their duty is to safeguard it. If the sick person recovers, he has the best claim to his wealth; and if he dies, each heir takes his share.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2