Some parents are convinced that epileptics are retarded and spend the majority of their time treating their epileptic children like brain damaged two year olds. There's nothing you can do about it. Neither education, nor evidence to the contrary will prove someone wrong who takes pride in ignorance, and is convinced of a hypothesis. Not even a high probability of being in error.

Here's my advice. Set up a direct deposit of the check from Social Security into her bank account.