An 80 year-old lady is no longer able to stand up due to problems with her back. She now usually performs Salah (Prayer) while not in the state of Taharah (ritual purification),
although she does sometimes perform Tayammum (dry ablution) and sometimes she will perform Wudu' (ablution). When she sits in (Part No. 8; Page No. 69) her Musalla (a place for Prayer), she combines her Salah, performing them sitting, and completing a full Salah. When she has a good day, she repeats her Salah.
If this old lady is conscious and aware, and she offers complete Salah sitting because she is no longer able to stand, the only thing that she is deficient in is that she is performing Salah while not in a state of Taharah, if she sometimes performs Wudu' (ablution) and sometimes Tayammum. It is therefore the duty of those caring for her to guide her to what is obligatory upon her in regard to Taharah and to help her to maintain it. If she has offered Salah with Tayammum when she was aware and able to perform Wudu`, she has to repeat the Salah that she performed with Tayammum. However, if she performed Tayammum because she was unable to perform Wudu`, her Salah is valid. If she was not conscious or aware of what she was doing or what happens to her, then she does not have to offer Salah or make up for the Salah she missed during the time she was unaware. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.