Learning to go along with whatever they’re currently obsessed with can be very hard, especially when it’s something crazy that you know can’t possibly be true.
I had to learn this the hard way with my DM when she became obsessed with the idea that she and her cleaning lady had taken my father’s dead body to a graveyard many miles away and just dumped it. This arose from a dream, muddled up with something she’d seen on TV the night before.
It was no use telling her I’d been there when he died, I’d been there at his funeral - she was utterly convinced and terribly distressed for around 48 hours. Even her C. Lady telling her over the phone that she’d never taken her anywhere in her car was no help.
Knowing what I learned later, I should have said, yes, I knew, but it was all right, he’d been properly buried now (he was actually cremated), so no need to worry.
Later, when I was wiser, and she became convinced for weeks on end that her sister had ‘stolen’ their mother’s house (luckily she never saw my aunt so couldn’t abuse her in person) I knew that even a signed and sealed statement from the Lord Chancellor wouldn’t have convinced her.
So I just kept saying (her short term memory was zero) e.g., ‘Dear me, that’s terrible, I had no idea - I’ll get on to the police/a solicitor first thing tomorrow.’
I repeated this over and over - it always pacified her for the moment.