Trying to convince people to change their belief is not easy, let alone if they have early signs of dementia. People are attached a lot to emotion and the model they have in their brain, whether these are true or not. Imagine the silly situation, in which you are live in England and think you are an Englishman for 60 years - then someone comes by and say you are an Irishman. Of course, a mental transition is not easy and deals with a lot of emotion. Avoid religious discussion, this is not your goal.
I think you goal is that money does not go from her to him.
There are various ways to deal with the situation. I have made a list of some options:
- ignore
- by reason: very difficult, because of the model in brain
- by emotion 1: use an imposter, make the money go to you
- by emotion 2: use another Christian to convince her not to do it
- by emotion 3: use some text in the bible to convince her. to make it seem magical, you could make it appear in the room
- tech 1: a custom DNS-server which forwards mail to you instead of him (MITM attack).
- tech 2: break the computer, format C:\
- tech 3: identity theft: use a sendmail tool, fake his from adress. tell you have a new email address.
- legal: you could find a law related to it
The best is probably the emotion approach, using another christian.
be careful and make the best of your time. Dementia is terrible, your mother is still here now