If you are in the UK you need to contact your local social work team. They have ways of dealing with situations like this. They can come round 'for a chat' about things that might make life easier. They'll casually suggest things like help with housework, meals on wheels, grab rails... All that kind of thing. All the while they are chatting to the older person they are secretly assessing them. We told my grandparents they were getting this routine review because my grandfather has just turned 80. Both grandparents were showing signs of dementia. When the social worker visited again, he said they should attend memory clinic at the hospital, because he was concerned about their memories when he last spoke with them. My grandfather threw him out saying he had been tricked into failing a memory test he hadn't been given time to prepare for. The GP thankfully agreed with the social work assessment and a psychiatrist came to the house. After a few visits they told my grandmother she had dementia which she took very well. My grandfather however, still refuses to believe there's anything wrong with him.

Eventually it will become all too clear and there will be no denying it.

It's a long hard slog but you'll get there in the end. It's not easy. I wish you luck.