My friend, no doubt influenced by dementia and paranoia he was feeling, changed the passwords, made no note of them, and subsequently died. The computers in question run Windows 10 using Bitlocker and key storage in the TPM.
The data is effectively gone. I believe he was using encrypted backups to a "cloud" storage provider, too, but I'm also fairly certain the key is only on these computers. (The Windows accounts on these machines are local accounts so the Bitlocker recovery keys weren't saved on Microsoft's servers either.)
Matters were arguably handled poorly on my friend's part prior to his becoming of unsound mind. He wasn't terribly technically savvy and I'm not sure he considered the "losing my own mind" threat model. Nonetheless, it adds insult to injury that Bitlocker, which added no security for his day-to-day use, effectively caused the loss of his data.