I agree with the other responses here but it sounds like you've already tried all the right solutions to convice him and that no one answered your actual questions...

I did quite a bit of research when buying my own safe. What I learned is that you want one that is UL tested for 60 minutes of fire. If it isnt actually UL certified, then it probably doesn't actually work. And everything you can find easily and cheaply won't be, sadly. If you buy online you will need to pay fright, most likely, too, which can be a hassle.

If he's only storing cash, then I suppose it doesn't need to be waterproof, but consider that the fire dept is going to be using copious amounts of water to put the fire out, also.

Most tests show fireproof bags are worthless. They are only even exist because you can put flammable things in them to prevent a fire from spreading. They won't protect what's inside at all, but they do protect the rest of the house from what is inside. Regardless of what they claim on Amazon. Don't even bother.

As far as I could tell there is no cheap solution, and really even with my own high-end safe I feel like it's a gamble that it will actually work. Makes me feel better but not great.

(And no, I'm not hording cash in it, before you all start pouncing. )