Did the friend err in informing his manager that the driver was not supervising the fueling of the bus, and should he be blamed if the driver is punished?

Your friend is not to be blamed after having reminded the driver of the necessity to check the fuel, for the driver is entrusted with the responsibility, and his negligence is considered dereliction of duty. If the driver is punished, there is no sin upon your friend. If the driver assaults him, your friend has the right to defend himself and report the matter to those in authority.