So, i'm going to present an opposite idea.

Big ass gun safe. Like, a 12-36 (rifle) gun safe. Heavy, fire & water resistant for 60-120 minutes. He knows the combo and you do. He dies, you can open it. Avoids the wife situation, consolidates the money into a "healthy" environment; relatively theft proof (esp if bolted down); still provides him the hard cash; Available to store important documents and precious metals if he changes his mind; able to store firearms as well; it's something that can be handed down or stays with the house for value.

My mom was a bit the same way, stashed cash all over the house. I was aware, but wasn't really behind it, it was whatever though. I joined the Army and ended up having to deploy. I needed somewhere to store my guns because I didn't trust the Army's storage at the time. So I bought, and had delivered & installed a really nice big ass gun safe. I used it during deployment. It's still at my parents house. There's no value in moving it. They now use it to store documents, some precious metals and other things. I doubt they would have bought it on their own, but are more than happy to use it now. I know the combo, and I have access if needed.