Is it obligatory to inform the manager about a driver who uses the company bus for personal purposes without his knowledge? And is such informing considered backbiting (ghibah) or instigation (fitnah)? And is the driver entitled to continue doing so under the pretext that other officials do worse?

It is forbidden to use a government bus for personal purposes without permission, and the perpetrator must repent. It is not permissible to commit a lesser evil because of a greater evil committed by others. The perpetrator of this evil must be admonished, and if he does not respond, it is permissible to inform the authorities or request his replacement with a trustworthy driver. Similarly, officials must be admonished for their evil deeds, and if they do not desist, higher authorities must be informed, because consuming public funds is a major sin.