Chattykathy, I know of a case like that. Fairly early dementia, but the man had not yet been officially deemed to lack mental capacity.
A live-in carer, employed by his close and loving family to look after him, very soon completely estranged him from them, took him abroad and married him, and got him to change his will and leave everything to her - and it was a substantial estate.
He died not long afterwards.
They went to court but the woman was so clever and plausible that they lost the case.
The person who told me all this strongly suspected that the so-called ex carer has hastened her father’s end - she was an ex nurse - but since she’d swiftly had him cremated there was no way to pursue this.
People with dementia are incredibly vulnerable to the unscrupulous, particularly if they come in a guise of ‘caring’. Families need to be very vigilant and preferably have a Power of Attorney in place ASAP - often easier said than done, though, I know, since dementia can make people very suspicious of others’ motives, not to mention being unable to understand that they’re no longer capable of managing their own affairs.