My grandpa was a depression era style horder. He didn't trust banks at all. He'd put his cash away in holes in the wall and it would literally rot or rodents would eat it. It was fairly easy for my uncle's to convince him that the American dollar has no intrinsic value and was really just as bad as the banks he was avoiding. My grandpa of his own volition chose to convert his wealth horde to rare metals which could be hid in a wall without rotting away.

Wrapping the cash in tinfoil will literally bake it until it's black. Fireproof safes and bags are rated for the amount of time they can protect the contents in case of a fire. It's not the object actually catching fire that you have to worry about, it's the heat.

Honestly there is no way to get through to people like this. If you want to achieve your goals you basically have to manipulate them by feeding into whatever it is they believe. My parents are old and they don't believe in inflation no matter how I try to explain it to them. It's a government hoax to convince people to lose their money to wall street thieves. Luckily, my parents don't believe that 401k are invested in the evil stock market, they think the 401k is a company funded pension. Just.... Fuck man, at this point I take the victory wherever the suspension of disbelief happens. Good luck.