He is certain that he is the only person who has seen his phrase.
The only thing now is that there is no proof to back it up.
OP In cryptocurrency, transactions are irreversible so there is basically no way to get the coins back.You can however make use of blockchain explorer to see the address that received the funds and confirm the transaction, but that’s the only proof you can get of where the money went, not the identity of the person that took it
We already have the address.