I think your best bet is exactly what you already figured out. Quietly take the cat and take it home. Is there any chance she would come over & see Tornado by accident?
Dementia, sadly, has no real treatment...so if she truly has it, she is likely to get worse. I agree that she will probably forget.
At the very least, she lives on a farm...and it would be easy to say that the cat just took off & never came back home for whatever reason.
I don't like the idea of being dishonest...but sometimes "necessity is a higher god than truth."
Thank you for your advice- yes, she will think he took off. There is another cat there, too, that I can't get near, and hopefully that one will keep her company. No, she will never see Tornado at my house, as she doesn't drive and never gets up here anymore. I think the plants I planted outside will keep her company, too...and will take less care.