My father in law hoarded about $30k in cash that was discovered around his house after he died, along with some silver. He was very distrustful of banks and the government which is why he did that. We never would have been able to convince him to deposit or invest that money for that reason.

Fireproof bags or safes would certainly be an improvement. Maybe it would be worth asking if he'd buy precious metals (gold/silver) instead. Even though it's not considered to be a great investment by most, it typically at least keeps pace with inflation, would allow his to keep his wealth in his home, and would hold up better against a fire, since the melting point of both metals is higher than that of a typical house fire (and even if it does melt, the metal can still likely be recovered).