How to stop my Dad from being mentally abused?

Your profile also states that your father is living at home; is that the home you share with your stepmother?

What are the specific allegations of being "financially destroyed?" How is this being accomplished? Are you stating that she's confiscating his financial assets? Is she named as attorney-in-fact in a DPOA?

More information and explanation is really needed to understand the issues here.

It would be helpful if you gave us more information. From your profile you wrote that your father has Alzheimer's/Dementia. How old is your Dad, and the age of your Step-Mom? Does your Step-Mom work outside of the home? Or is she the primary Caregiver, depending on what is your Dad's current needs. I doubt after 37 years of marriage she would be considered a gold digger.

What do you mean my mental abuse? What is it that your Step-Mom does that you could be saying this? Is it a case that she doesn't understand about Alzheimer's/Dementia? That diagnoses can be down right frightening to a spouse, and she could even be in denial, or trying to correct what your Dad does or say because she doesn't know what else to do.