As others have said, the insurers will have no interest in paying out for anything other than the immediate damage, and as they only have a responsibility to 'reinstate' then any arguments about compliancy tend to fall on deaf ears. I've been in that situation myself with an electrical claim where I was the insured customer, so I know this chapter and verse. The best outcome for the owner is probably to get the insurers to pay for your EICR (as that's justifiable), you quote for the direct 1 for 1 insurance work and then send the customer a separate quote for a complete rewire (and expect it to disappear into the round filing tray shortly after).
Lastly, I've also worked with a lot of hairdressers over the years and they're not the brightest candles in the box except when it comes to making money so keep it simple and don't expect too much!!