She is stealing and taking advantage of him. But here is the catch...she has him convinced that it is his children that are doing wrong. He will not go to a Dr appt without her being there and she is the one that speaks for him. Last time I said something to him about how I felt he wouldn't talk to me for about 6-7 years.
If you are holding back and not giving him prescribed meds, I hate to say this, but you will be charged with elder abuse. Tread carefully.
Health insurance companies monitor pills now a day, they know exactly how many pills are going out and when the refill is coming in. If something is out of whack, the doctor will receive a notice from the insurance company.
Instead of working against this woman who is there taking care of your Dad, why not work as a team.