Mom's 84 and does not want to shower or bathe, all she wants is sponge bathing. Advice?

My mother-in-law did that for a decade, and at the time she DIDN'T have dementia. She was just afraid to fall in the shower is all. She takes a shower now, since we got her a shower chair that spanned the distance between the toilet into the shower so all she had to do was go from the toilet and scoot right into the shower. Of course now she's in asst. living because she DOES have dementia now, but still uses that shower chair! She never did stink, so none of us ever knew she was doing the P.T.A. method (pits, tits, ass) and my father-in-law sure never told us. Oh well.