I get what you saying but you can find many posts detail that. requirement. I had to do the same. Which embassy is he applying from,?He can try submitting without it, if they want it they'll ask for it.
Focus on detailing his work online, business registration, screenshots of websites, LinkedIn profile etc. Letter detailing permission to work remotely in Thailand and with no Thai clients.
You should know business class flights, 5-star hotel bookings are not mandatory requirements for entry to Thailand, is the point. I do not think i know better, im just giving you a fact about entry.