What should I do if I suspect Mom, with Alzheimer's, is being taken advantage of financially, by her husband, my stepfather?

I suspect my stepfather is taking advantage of my Mom's complete compliance and inability to have any kind of defense system to question his actions. Their arrangement is to split the bills, but she ends up paying the large bills and he pays the small cable bill, and has even skipped his month to pay that twice and she pays double on her month, without even knowing. It’s happened more than once..She pays for all the gas for the car, car repairs, groceries, tax prep,etc, etc, etc...When they go out to eat, he pays most times, but they split a meal. They eat breakfast at home and get lunch from the nursing home 5 days a week. Yes, he has been mostly a kept man for their 20 years of marriage, but now she can’t reason for herself..And he has money from the sale of their home. (His home), that he could now afford to take care of her a little more.
It’s getting completely out of control trying to piece together the puzzle when I go to their home, 100 miles away, once a month.  Talking with Mom does not one bit of good, since she won’t remember when we hang up. Her husband is her main caregiver and she has not invoked the POA , me, that was drawn up over a year ago, saying she’s not ready to lose control. But In fact, she has no idea of her bank balance-on a monthly basis, even though she gets paper statements. She disagrees with the Neurologist diagnosis that she needs someone in charge of her financial and medical decisions.  I feel helpless to help protect her. Any suggestions?