They should buy a Yubikey (a physical key) and keep their passkeys resident on that. Then they can truly keep that physical key locked up in their safe. And it is truly safer than their physical notebook because they are safe from being tricked into entering their password in the wrong place.
How do they tell their significant other how to logon to a shared account when they aren’t together? How do they use the Yubikey with their phone, laptop and tablet conveniently?