Is it permissible for a cancer patient undergoing chemotherapy to use nail polish, knowing that nail polish invalidates the ablution (wudu'), and is this considered a necessity?

If nail polish is necessary to prevent nail breakage or damage, and there is no alternative, then it is permissible to use it out of need. Wudu is valid with it on, and the polish takes the ruling of a splint (jabeera) in terms of wiping over it. We give the sister glad tidings of good and advise her to be patient and content with God's decree.