I’m going to go with giving him some helpful advice, and offer of help if you like, but point out the incredible dangerousness of the arrangement. Planning may or may not care about his lack of permit, but if a fire marshall shows up, the place will be red tagged as uninhabitable, and that’s it. Of course you might need to call the fire marshall yourself. But seriously, that sounds like a major disaster on the hoof.
Selling the house may be a lot more difficult with home modifications that are seriously noncompliant with code. He may end up eating the cost of upgrading the property anyway.