I assume their code base has been working for 10 years? Not every owner cares about scaling or quality, sometimes speed is their selling factor.

That's why we have McDonalds and 5-star restaurants instead of only 5-star restaurants.

You can try to educate him on why documentation would help speed up development, would make on-boarding new devs easier/faster (increasing product quality/stability and possibly decreasing churn for when your critical API fails on a customer...)

But his business is clearly working for him. So why change things? Why spend a lot of time (and time=money) on fixing problems that haven't ailed his successful business for 10 years?