Please have a look at the Alzheimer's Society website. There is a very helpful forum called Talking Point. Whatever you are going through, others will have been there - not that there are often easy answers. Many people inc. me have found it a lifeline (mother and FiL both had dementia).
Except perhaps in the very early stages, the very fact of having dementia can often mean that people can't understand or remember that there's anything wrong with them. They can't remember that they can't remember anything, if that makes sense.
My own mother was told by her GP that she had Alzheimer's, but had forgotten by the time she got home maybe 10 minutes later, and reminding her only made her cross - there was nothing wrong with her! - so we soon stopped, and just referred to 'getting a bit more forgetful' instead. After all, nobody wants to be endlessly reminded that they have a horrible disease with no cure.
If a person isn't very reluctant to go to the GP at all, what is often recommended is to write to the GP explaining the circs, and ask for the person to be called in for an 'over X age health check'.
Good luck.