To sue the employer, your husband would need to show that the employer had been negligent.
If, for example, the vehicle air bag failed to operate (and there was no reason for the employer to suspect that it might be faulty), the employer could not be held to be guilty of negligence.
On the other hand, if the employer was aware of safety defects with the vehicle, and failed to rectify them, they were negligent in requiring your husband to drive the vehicle. (However, if your husband was also aware of the safety defects, a court would probably rule that part of the negligence was his own, as he should have refused to drive it. Indeed, if he agreed to drive a vehicle which he knew had insufficient safety provision, he was committing a criminal offence under the Health & Safety at Work Act, by endangering his own safety).
Chris