The real question before the quick answer is what hardware do you have?
Even more to the point, he said "just need something to work with the mechanical equipment it controls."
So what exact mechanical equipment does he need to control?
If there isn't any off-the-shelf software for that mechanical equipment for a particular OS, it may not be straightforward to do so. Especially because he said he's not an IT guy.
In cases like this, the best choice might be to pick a stable OS that has the software, and make sure it's air gapped, so it won't receive OS updates or other things that can break the system.