This is BitLocker encryption, not something I can call about. The recovery key is tied to whichever Microsoft account originally set up the device, not the current owner.
Try this first: go to account.microsoft.com/devices/recoverykey and sign in with your own Microsoft account — if it was ever linked to you, the key will be there.
If it was tied to his account, only he can retrieve it.
If neither works, the device will need a factory reset, and the old files likely won't be recoverable.