@Davis *****
If you read the fine print, non IM B is for visiting businesses, it is not a work permit allowing you to work freely in Thailand. To work under that, you need a work permit which requires sponsorship from a Thai company.
I just signed the mountain of paperwork for a work visa just so I can interface with the factory we want to work with here. When I called immigration to ask if I could attend meetings regularly under the DTV I was told absolutely not, and that I would need a work or business visa. I work remotely from Thailand, and past contracting factory work here we don't do business. But that's enough for them to want you on a different visa.
He wants to work in Thailand, even if his business is based elsewhere that business is being conducted here. Pretty clear grounds to either avoid and stay on the DTV, or change over to an appropriate business/work visa.