This is baffling. It would be remarkably easy to memorize 12 words, then keep no active wallet anywhere. When you want to send coins, start up a desktop wallet, enter the 12 words, send some Bitcoin, then delete the wallet.
If this was a really old Bitcoin wallet, it could be from before the "12 words" thing. IIRC, older wallets randomly generated each key, instead of deriving them from a seed passphrase (which also meant you had to be really careful with your backups, since an old backup of the wallet wouldn't have its newer keys).
I am sure you’re right. But you could just send the funds to a new address in the new wallet. He could have done that at any point in the last 5+ years.