بسم الله الرحمن الرحيم
(Fatwa: 1122/1205/L=11/1437)
Tayammum is valid only when the use of water is feared to cause or increase the sickness and it is confirmed by physician or by experience. If it is not so, then tayammum is not valid. You should make arrangements for your mother to do wudhu on floor.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband
