I guess the short answer is that an employee is always protected (except in the case of negligence on the employee's part) by the employer's liability insurance, as long as the employee is following the directions given by his supervisor.
however,
There are about 4 thousand other details that would make all the differences in the world about this: what was said exactly, and when was it said, what was the failure and how did the repair somehow differ from "industry standard" repairs, and expert opinion related to should the tech have been able to predict the failure, and how having the manual or other information would have changed the outcome and injury or the repair procedure.
Someone getting hurt 2 weeks after a truck was touched by some service tech does not make the injury the fault of the service tech, with or without the manual.