Bathing Assistance Issue

My 91 year old mother-in-law has lived in an apartment under my house for 8 years. She has always been very easy going and still often is for the most part. She has been experiencing memory issues for a couple years, but her memory has declined significantly since January. She was diagnosed with early stage Alzheimer's in March.

I am increasingly helping her with many things. She is still able to live independently with help, although her memory decline has been picking up speed in the last few months in spite of being on Aricept and receiving memory therapy.

The problem is that she needs assistance now with showering and hair washing. My MIL also has severe osteoporosis and uses a walker. This problem along with her memory issues are making it impossible and dangerous for her to shower independently. We have a woman come in once a week to help her. My MIL won't allow the woman to help her more than once a week. The woman has done an excellent job and is very kind to my MIL. My MIL likes the woman a lot but absolutely hates receiving help with bathing. She complains to me constantly and argues that she does not need her help. I then have to tell her why she needs the help. After several exhausting minutes, my MIL finally admits that she can't bathe on her own and that she knows that she has to accept the help. However, her short term memory is deteriorating, and she forgets our conversation and starts arguing with me again within a few days. It is getting exhausting because it is turning into a weekly thing. Thank goodness she is always nice and cooperative when the woman actually comes to help her.

Next week, the helper has to change her day, and my MIL told her to skip next week. That means she will go two weeks without bathing or washing her hair. She thinks her wiping herself down with a washcloth is enough, and she could not care less about washing her hair. She is satisfied only getting her hair washed every 6 weeks when I take her to the hairdresser. This is a woman who used to be very careful with her cleanliness.

I know it's the dementia and her frustration with needing assistance, although she welcomes my help with other things. This is clearly not the person she was even a year ago, but what do I do? I feel like I am going in circles with her on this subject. Suggestions would be appreciated.