Well, my advice would be get them to contact the installer to get them to sort it out, mind you If it was left in that state I would not hold my breath for too long.
As to all the other avenues that are supposedly available for the customer, well, they might as well piddle into the wind, they will just get frustrated being sent around in circles.
You have seen the job, it does not look too difficult to sort it all out properly and split the system over the 3 phases, that is if it was installed under G83 that is, also how is the supply used at the property, do they use 3 phases or just the one?
If you are happy to do so, offer to sort it out properly for them, for a price of course, but make it clear that you are not guarantying the system for any future faults.
If you do contact the DNO, you do stand a risk of them coming to look at it which could end in disaster, they could shut it down and remove FITs too..
Let the customer decide, but point these things out to them.