[dr_zoidberg]

First and foremost: I AM NOT A LAWYER.

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.