What can I do if my demented father wants to take away my POA and give it to his demented girlfriend?

My 96 year old father is in declining health both mentally and physically and is living in Florida under the control of his 83 year old girlfriend who was diagnosed with dementia in 2009 but fired her Dr. and refuses to acknowledge it. She writes the checks from his checkbook and has been draining his resources for years. He is running out of money and she is trying to get at the money in our mother's trust which we know they can't legally touch. Because his girlfriend’s dementia is worsening, we convinced my father to give my brother and me both durable and medical POA over him. His girlfriend is livid and is harassing him to sign it back to her because she doesn't want us to be in their financial business. He is afraid of her and says he wants her to make her happy and that he’s sorry he gave it to us and is thinking of signing back to her. Does it make sense for us to get an attorney to write her attorney a warning letter putting him on notice that both of them are of questionable mental capacity so they better not be a party to signing the POA back to her?