Topic 5567641
What prevented him from keeping a "metal" backup (even if it's paper)?

Or maybe he should have used an HDD (given its sensitivity to shock), which is likely more durable than USB flash drives.

He didn't use paper, because as I said he wanted to encrypt the wallet. Which is not an excuse, because he could have used a BIP39 passphrase or a BIP38 encrypted paper wallet.

I discovered another "ticking time bomb" in this story. Did your friend store his passphrase at the bank? Does that mean he trusted third parties? This could also be considered a vulnerability that could lead to loss of access to the wallet.

It's not really considered a bomb. I wouldn't do it either, but as far as security is concerned, having a passphrase in the bank isn't going to cost you a lot. Let me clarify:
- a passphrase alone can't cost you any money. Anyone having it, needs to have physical access to a seed phrase backup, otherwise, it's just a random set of words or characters.
- a bank's vault can't be opened without the physical presence of the owner. *

* It can be violated if the bank is "legally" enforced to violate it. It can also be violated during a potential robbery. And more importantly LoyceV had written a post** once, showing how easy it is to violate a vault's lock without even leaving a small trace.

** Sorry I really can't find it now...