My Great Aunt was a well travelled, well informed woman and after being widowed early managed all of her finances, investments and house sales/purchases at a time when her background would never have equipped her to manage.
At 103 she was unable to look after herself physically but still very lucid, and had to go into a home. Well meaning but less educated members of the family would not tell her the truth, and often were telling her different things, to the extent that she was absolutely confused, and could not understand what was happening to her. So I would say tell the truth gently