Praise be to Allah.
The ruling concerning your father differs depending on his state of mind. If he loses his reason and his memory then he should not be told to purify himself or to pray or fast, but when his mind is present and focused and his memory is with him, then he should be told to do the duties enjoined by sharee’ah as best as he can, without causing himself undue hardship.
Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allaah have mercy on him) was asked:
Do those who have lost their memory, the feeble-minded, young children and the insane have to fast?
Allaah has enjoined acts of worship on man if he is able to bear that obligation, by being of sound mind and able to understand things. But if he is not of sound mind then he does not have to do those acts of worship. So the insane and young children who have no powers of discernment do not have to do these acts of worship. This is by the mercy of Allaah, may He be glorified and exalted. The same applies to the feeble-minded person whose mind is affected in some way but not to the degree of insanity and the elderly person who has lost his memory, as this questioner says, do not have to fast, pray or purify themselves, because the one who has lost his memory is like a small child who has no power of discernment, so the duties of Islam are waived for him and he does not have to purify himself or pray, nor does he have to fast. With regard to the financial obligations, they are required of his wealth even in this case, so zakaah – for example – must be paid by his guardian on his wealth if it reaches the amount at which zakaah becomes due, because the obligation of zakaah is connected to wealth, as Allaah says (interpretation of the meaning):
“Take Sadaqah (alms) from their wealth in order to purify them and sanctify them with it, and invoke Allaah for them. Verily, your invocations are a source of security for them; and Allaah is All-Hearer, All-Knower”
He did not say “take it from them.”
And the Prophet (peace and blessings of Allaah be upon him) said to Mu’aadh (may Allaah be pleased with him) when he sent him to Yemen: “Teach them that Allaah has enjoined on them sadaqah (charity) from their wealth, to be taken from their rich and given to their poor.” He said, “Charity from their wealth” and explained that it is to be taken from their wealth, even though it is to be taken from the owner of the wealth.
Whatever the case, the financial obligations are not waived for a person in this case, but the religious obligations such as prayer, purification and fasting, are waived in the case of people like this man, because he does not have any powers of understanding.
Majmoo’ Fataawa al-Shaykh Ibn ‘Uthaymeen, 10, question no. 40.
The Shaykh (may Allaah have mercy on him) was also asked:
What is the ruling on fasting for one who is sane sometimes and insane at other times, or who is incoherent one day and focused another day?
The ruling changes according to the reason. At times when he is conscious and aware, he has to fast, and at times when he is insane and incoherent, he does not have to fast. If we assume that he is insane one day and sane another, or incoherent one day and focused another, then on the days when he is focused he has to fast, and on the days when he is incoherent he does not have to fast.
Majmoo’ Fataawa al-Shaykh Ibn ‘Uthaymeen, 10, question no. 43.
And Allaah knows best.