No its not his fault, you sold him the account, the account was locked and therefore its your responsibility to unlock it back.
Ive told you my situation here. There is no point in repeating what ive already said. The date of birth was random and there is no way i can tell him. The account was his own, not mine ( cause he changed almost all details after i gave it to him. I dont know whether he has/hasnt changed the date of birth ( cause he had full access to the account ).
Thats my final point. Id help him recover it if they wanna know my name or email or address used(That was the real info attached)