Health- Dementia?

Hello, I have an important question that is bothering me and my siblings a lot. My father is 84 and we can see how he is changing more and more. It is heading towards dementia. He has a girlfriend who is 62 and we can also watch with great concern how she is taking advantage of him. But the most important thing to know is that my father really thinks he is very fit. Physically he is, but we think that he will soon no longer be able to do business. What can we do to get dementia confirmed by a doctor without hurting him? He has already had an appointment but refused further tests.