All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
If there is a disturbance in the mind due to old age, it can either be continuous or intermittent:
If it is continuous, then the obligation (of performing prayers) is lifted from the person, and prayer is not obligatory on them at all due to the absence of the essential condition for the obligation, which is sanity. Al-Imam al-Zurqani, may Allah have mercy on him, said about the conditions of the accountable person: "The fourth condition is sanity. The insane person is not accountable by scholarly consensus, and his accountability is impossible because he does not comprehend commands and prohibitions" [Al-Bahr al-Muhit fi Usul al-Fiqh, Vol. 2/P. 62].
If he occasionally regains consciousness and can focus during part of the day, and he becomes aware that the prayer time has entered and that it is obligatory upon him, then he should pray in the best way he can in his current condition. Allah, the Most High, says: "So fear Allah as much as you are able" [Al-Taghabun, 16]. It has been mentioned in the book [Al-Iqna' fi Hall Alfaz Abi Shuja', Vol. 1/P. 131]: "The prayer is not waived from him as long as his sanity is intact, given the existence of the cause of the obligation."
Therefore, if the asker`s mother is unaware all the time, then she isn`t required to pray in the first place. However, if she wakes up at some times and is unaware at other times, she should perform the obligatory prayer of that time in the best way she can when she is awake. If it is the time for 'Asr, she should also pray Dhuhr because the time for 'Asr prayer has entered. Moreover, according to Islamic Law, the time for 'Asr is considered a necessity for Dhuhr. This is evident from the fact that if one combines Dhuhr with 'Asr where he delays their performance to the time of 'Asr, it is considered valid (As if Dhuhur was performed at its normal time). Since she caught the necessary time for Dhuhr, she is obliged to pray Dhuhr. The same ruling applies to the 'Isha prayer, provided that she remains sane for a period that enables her to perform both prayers. For more details, kindly check {Moghni Al-Mohtajj, Vol.1, P.315}. And Allah the Almighty knows best.
