Tell the Restaurant owner to call the Insurer and inform that they will be receiving a letter from his solicitor demanding the claim be settled within 14 days or they are in breach of contract, at that point he will instruct his solicitor to compile paperwork to request a Judicial Review of their decision in the High Court.
The Insurance company will s**t a brick in all likelihood because they know that the majority of people are unaware private individuals are able to do this for this type of problem and are scared off by huge legal bills. They will know that a Judge will review the details of the case and then decide to either hear the case themselves or call a panel of three Judges if they feel it warrants it, as as this would have such a massive impact I bet they would. The decision of the Judges will be final in all reality because they are clearly in breach of contractual law by insisting on an NICEIC registered company because they do not have that right, they can only insist on a company that is certified by an acceptable trade body, and NAPIT is most certainly that.
Don't take prisoners and blow them apart.
For what it's worth i had a problem many years ago with a London Borough who refused to give me a permit to park my car outside my home, because I was appealing a yellow box ticket after I was stranded to make way for an Ambulance on Blues and Twos. After thousands in tickets had accumulated and they towed the car away numerous times I was advised to go down the route above..
I won £104,000 in costs, loss of earnings, compensation and "mental anguish". The Council not only lost, but three of their staff were so scathingly attacked by the Judge that they were sacked, including the parking manager, and two managers from NCP who managed the parking and the pound services for the council.
Little Fish can win against sharks if you know how to hit hard.