The owners of the forklift and the employers of the forklift driver are liable. It becomes a little more complicated on private ground, but ultimately they are liable. He needs to contact their company and request their public liability insurance details, as the forklift may not be registered or insured, though some are. Did he check to see if had a registration plate on it? If it did, it may be seen as being an integral part of the tractor unit and trailer if is attached to it for transportation. If so, then it will be on the vehicle fleet insurance. Should the company responsible not provide their details, he can either claim via his own insurance and let them sort it out or claim via the courts, but he would need to evidence his loss by having an independent expert witness/engineer inspect the vehicle.