How do you alert someone their loved one likely has dementia?

Someone I am tied to both by blood and for many decades financially. Over the last couple of years is showing an increasing amount of Alzheimer's and vascular dementia symptoms (drastically increased in the last few months), along with other health ailments that may have contributed to the vascular dementia component. From balance & movement issues, delusions of theft (which are increasing in magnitude), erratic driving, and some forgetfulness/confusion. My issue is the delusions of theft are aimed at myself, "that I have been thieving from our joint business dealings". While we are pretty much retired and let separate business, audit and legal professionals manage we do keep an eye of the monthly accounts, to which I haven't seen anything untoward since our retirement. I went as far as matching all expenses, receipts and income to the cent. I also had a professional audit with the same findings and in turn now both myself and the person that did the audit are "thieves". My partner's loved ones prefer to believe the delusions (even though over the decades of our business partnership there has never been a problem of financial entitlements for each partner) than to face the likelihood of a debilitating illness such as dementia. As I am now the "bad and evil one" convincing his loved ones to seek professional help will only make me a bigger "bad guy". Mutual acquaintances have tried to speak and reason with my partner and they too have been thrown into the "thief" bucket. I fear my partner's actions of recent months will bring about financial doom for both of us. I don't know what to do. Selling out is also out of the question in this current climate.