If they didn't lay claim to the IP, you should polish it and offer it for sale, or use it as interview material at someplace developing similar technology.
They could argue that since it was a tool that was made to help with his job at at the company, then it's internally developed. If there are no clear grounds or easily presentable evidence (even if there was!), he's out in the wild with a liability.
The only place where I can see him in the clear is if they had a repo going back to before the work on the company and they could prove the tool was a personal project, and unrelated to the job. Even then, Company could still sue and burden them with legal fees/process until they tire him out and BINGO, now they own their IP.
The alternative could be starting a new project, completely open source from the start (probably with one of the more liberal licenses) and get crowdfunding to develop and maintain it. Assuming they are interested in doing that, of course.
Oh the creation of modules has gotten much better with languages like SystemVerilog and SystemC :) . What I did was a very rudimentary effort in perl (later ported it to python). Kind of like people look like "excel gods" at some company when they take some mundane manual tasks and do some excel scripting and take a 3 day job down to 15 minutes. I just happened to be the only EE with the soul of a coder while I was there. I was under very heavy NDA at the time, I'm sure others were doing the same at their companies. tldr; it wasn't anything special and is much more common these days. Basically it eliminated a ton of copy-pasta and manual editing.
With that out of the way...
They could argue that since it was a tool that was made to help with his job at at the company, then it's internally developed. If there are no clear grounds or easily presentable evidence (even if there was!), he's out in the wild with a liability.
The only place where I can see him in the clear is if they had a repo going back to before the work on the company and they could prove the tool was a personal project, and unrelated to the job. Even then, Company could still sue and burden them with legal fees/process until they tire him out and BINGO, now they own their IP.
The alternative could be starting a new project, completely open source from the start (probably with one of the more liberal licenses) and get crowdfunding to develop and maintain it. Assuming they are interested in doing that, of course.