You should read about relatives trying to help victims of romance scams on reddit /r/scams. Generally its hard or impossible to convince some who is emotionally invested. I recommend you point out past BTC volatility and drops, and suggest he diversifies a bit if that’s all his savings. He could put some in gold, some in shares, leave some (maybe 50%) in BTC. He will still win big if it “goes to the moon”, and will have some protection from a crash. Often you need to make a mistake before you can learn this lesson. Eggs in baskets.

Durable tools of all types that you can use or sell are good value in an uncertain future. Useful skills and community are important as well. Lynette Zang talks about all this.

The first 3 letters are unique for the English seed words, so you just need to encode 25*3 = 75 letters.

I also think it is worth writing them down in a way that is not obviously a crypto address. You can place 75 letters in a 9x9 square (with maybe some extra dummy letters) that looks like a common word puzzle, eg sudoku or crossword, in a specific order, eg a spiral. You can custom print a template or copy one from eg https://puzzlestream.com/sudoku/blank-grid.php .

The US dollar has lost about 98% of it’s value since 1971 yet people still use it in trade rather than gold or silver or a better fiat like Swiss Franc. Don’t underestimate the power of inertia. Just because USD has inflation problems doesn’t mean XMR is about to take off. It will take hyperinflation or a currency crisis and even then better-known cryptos like BTC or LTC will be the first choice for normies.