Who is this person attempting to control your dad? Is she a relative? Does she have power of attorney for him?
I would think you as a daughter would have more say to continue supporting him and his decision like you said.
My mom wanted to die in her own house not at a facility. And she did. She died in her sleep in the home she’d lived in the past 50 years
The person is Dad’s step-daughter. They have a good relationship and she treats him as if he were her actual parent. She just interprets him through her own worldview.