sometimes licenses are different between working for yourself vs someone else. Other times your employer can't take on jobs without you and your license present.
Then there's insurance differences between self-employed and employed ______ tradesperson.
From other specialist non-trade business I'm familiar with, people are normally aware of new employees coming to town before they even start working. People talk to each other.
> Get on Yelp, ask his customers to leave good reviews, and build a business.
You can’t do a good business without leaving visible traces - permits, customers, suppliers, etc. all provide ways for information to leak. If his employer is exploitative, he has above average odds of correctly fearing retribution and depending on the area that could mean unemployment. It’s not exactly uncommon for companies to discretely agree not to hire certain people.
This is the key reason why for most jobs, small town America is flyover country, an economically inefficient, exploitative, broken system. Without a large labor & employment market within easy commuting distance, your skills aren't portable if you get fired, and your boss can't readily find a replacement, so most employment relationships end up in a reluctant, often abusive state from one or the other.
This is why the normal trajectory, for a skilled tradesman, is apprenticeship -> experience -> accumulate clients -> go independent.
If you want to go independent in something like software you need, among other things, a good idea. If you want to go independent in the trades, all you need is a good reputation and enough people with your card. You don't even need to try either really. Do a good job and people themselves will proactively ask for your contact information. Finding a good and reliable tradesman is not easy.
> There are a ton of niches and other opportunities. For example being available during late hours. In anything like a large or medium city you can charge $150-350 per hour and people will happily pay it.
> Doing this while working for someone else is a good way to survive the initial ups and downs. If he builds a reputation for doing good work his clients will start begging him to quit his day job.
Did you think about possibility that his current employer will crush him like an egg for competing with him on weekends?