That applies to local shops as it does open source projects.
I've been working on Open Source software for 30+ years. There's no money in it, if your idea for making money is "accept donations". I don't like it, but it's a fact. If you want to make money, you have to make something that isn't free (and even then, if you give away the most valuable parts, as in "open core" licensing, you probably still won't make enough money to make the development worth it).
When I was young and driven by idealism and optimism, I assumed that with enough users I'd be able to ring the cash register somehow. Turns out not so much. We got the users, the money never came. There are a few outliers, but there probably aren't a lot of opportunities to found a Red Hat today.
Companies need to jump through legal and accounting loopholes to donate, they very much prefer a simple purchase, which is nice! But setting up actual purchases is a whole different ordeal with open source, now the question is why is the company paying for something that's free?
Source: my own 5-stars open source project with 500k+ active users that paid for 3 coffees in total over 10+ years. I still get like $2 sometimes after a long while.
Obviously, all contributors have some form of copyright, which may or may not have been waived depending on whether there was an ACL in place and jurisdiction. So he would need to get permission from the copyright holders, maybe in exchange for a percentage of the profit.
But it's MIT license. We can open a company tomorrow, take that code, and start selling it. Further development and improvements of the code could be trivially done openly or behind closed doors. FWIW the author themselves could do that if they wanted.
It's a bit of a niche as it is, so that's going to be rough in any kind of pricing model, as a large part of that niche is either homebrew types and the other commercial industry that will likely require some more integrations and customization.
I suggest GPL or AGPL because their copyleft clauses make them hostile towards platform providers who might otherwise seek to profit from your work without paying.
As long as they keep releasing sources of their own modifications -if they ever do any-, the rest is fair play.
Not all open source licenses are copyleft licenses (e.g., MIT very much isn’t), but at the very least copyleft licenses make it much harder to exploit open source code commercially without giving back in some way, whether that’s code, or cash for a commercial license.
Not perfect, by any means, but definitely an improvement over more permissive licenses.
I am aware of how much I’m starting to sound a bit like RMS in my old age.
In this case the provider would of course have to comply with the AGPL and release their modifications as you mention, but it's important to note that No FOSS license protects at all against, for example, just offering the code as a service. It's the exact reason why Mongodb changed licenses and then a stream of commercial products started to change into "Source-Available" licenses in the recent past.