Someone being? We weren't shown anyone to be in contact with the younger Denji other than the yakuza. The burial and the grave may as well have been handled by the yakuza to make sure they can trick Denji into paying his dad's debt, which he isn't legally liable to do. Denji didn't go to school, so he has no friends or teachers to reach out to. Basically, Denji is an uneducated, dumb recluse who only knows his dad and the yakuza as authority figures, so he believed them.