validty of prayer of person w Dementia

Assalamu Alaikum, My mom is diagnosed with Dementia. Lately, she started to forget how many times she performs her sujud and ruku, My sister who is taking care of her sometimes stop her while praying and let her perform again, sometimes she doesn't because she pities her. Questions: Is my mom's prayer valid? Should my sister not stop her in the middle of prayer? Should my mom prays again (we are afraid, it will stress her more)? May Allah reward you