The principle of vicarious liability holds employers responsible for the actions of their employees carried out in the course of their employment. Even if an employee breaches a company rule, the employer may still be liable if the employee was performing their duties at the time of the incident. In this case, although the driver breached the company's prohibition against having non-employees in the vehicle, he was still engaged in his work duties when the injury occurred. This situation is similar to the case of Rose v Plenty [1976] 1 WLR 141, where a milk delivery driver disobeyed company orders by allowing a 13-year-old boy to help him, and the boy was injured. The Court of Appeal held the employer liable because the injury occurred while the driver was performing his work duties. 

Key Point: Employers can be held vicariously liable for the actions of their employees, even when those actions breach company rules, as long as the employee is performing their work duties. This principle prevents employers from avoiding liability simply by prohibiting certain actions.