I think the problem you have experienced is that you started a project with someone else in a team of 2. You managed him as an "employee" but he felt (right or wrong) to be a cofounder. It seems you haven't discusses this prior of the project, so somehow you got stuck with a co-founder (and everything else that followed is just that).
I've been in similar situations, where cofounders have to split. It's never easy and there's always intensity involved. I'd not call them harassment though. Following again your company analogy, when you fire an employee, there is (often) drama - especially if you try to fire an employee without severance (well, in that case often there are legal problems).
Anyway, if you point out he was a volunteer at a startup weekend, do not mention the company analogy. In the company analogy - regardless if he was a cofounder or an employee - he would not walk away empty handed.