She is unable to use water. When is tayammum prescribed?

Whoever cannot do Wudu with water has to do tayammum and pray with tayammum. Tayammum (dry ablution) means striking the palms of one's hands on the ground then wiping the face and palms.

 It should be noted that Tayammum is prescribed by Allah when there is no water or when a person is unable to use it because of sickness and the like. Allah says (interpretation of the meaning):

 “O you who believe! Approach not As‑Salaah (the prayer) when you are in a drunken state until you know (the meaning) of what you utter, nor when you are in a state of Janaaba (i.e. in a state of sexual impurity and have not yet taken a bath), except when travelling on the road (without enough water, or just passing through a mosque), till you wash your whole body. And if you are ill, or on a journey, or one of you comes after answering the call of nature, or you have been in contact with women (by sexual relations) and you find no water, perform Tayammum with clean earth and rub therewith your faces and hands (Tayammum). Truly, Allah is Ever Oft‑Pardoning, Oft‑Forgiving” [al-Nisa’ 4:43]