I think it's less that he doesn't trust his wife, and more that, as TFA says, "many countries's laws make it difficult or impossible for a court to order you to turn over your keys; once the passphrase is known by a third party, its security from legal attack is greatly undermined, as the law generally protects your knowledge of someone else's keys to a lesser extent than it protects your own."
Like many security measures, he does not have a technological problem, he has a social problem. Namely, he is the lynchpin of his own security and he is very bad at estimating risks.
It is an absolute certainty that he will eventually die and his data will eventually become totally unavailable.
It would take an extraordinarily speculative sequence of events for him to run afoul of the British authorities who would then, being naturally covetous of the secrets of minor authors, abandon their tradition of spousal privilege and sue his wife for the contents.
Realistically I think he has a problem greater than either of these: assuming every bit of his data passes to his heirs as planned... then what? Does it have a comprehensible filing system? Does it have a document saying "Hi honey, if you're reading this I'm gone, the documents you need today are in ..."? And, the biggest question, is the data in there meaningful to anyone but him at all?
It doesn't protect her from people with a '$5 wrench' that want to get the password from her. This is assuming that 'they' figure that getting the password from her is easier/better than getting it from you.
Hired goon: "Excuse me, madame, but if you would be willing to divulge to me your husband's password, I will gladly give you this shiny new $5 wrench!"
(With apologies to "how to determine the height of a building using a barometer"...)
