This is just NOT a big deal! I could care less if he has a license or not. He is doing plumbing work and he is doing it legally.
My husband is licensed to install/repair irrigation systems.
You can have all the EMPLOYEES you want operating under YOUR LICENSE.
No biggie.
If Joe wants his own business, he will need to be licensed.
I can’t see what all the fuss is about him not having a plumbing license. It varies from state to state as to whether an owner needs to be licensed. In my state there is usually only one plumber licensed in a company, the rest are called apprentice’s. They do the same work without the master plumber license. No big deal.