Unable to perform ghusl for health reasons: Perform tayammum and not wudu
Answered by Shaykh Sohail Hanif, SunniPath Academy Teacher
Now if she is able to make Wudu can she make wudu instead of doing Tayammum. From my understanding she had surgery on her foot and had a bone removed. When told she should wrap the foot up and cover it so that water doesnt get to the wound, she said that the doctor told her not to do that nor to take a shower.
In the sacred law, if one is unable, for whatever reason, to perform a purificatory bath (ghusl), one performs a tayammum (dry ablution) in its place. A wudu is never a substitute for a ghusl. If one has wounds on ones body then one may leave the purificatory bath and perform a tayammum in its stead if the wounds cover more than a half of ones body or are in a part of the body where it would be impossible to perform a ghusl without getting it wet and causing harm. (al-Hadiyya al-Alaiyya, 37, Muhammad Said al-Burhani)
And Allah knows best.