I don't know of any place where there's severance when someone "voluntarily" resigns.
It's kind of weird when someone chooses to resign in the face of pressure to do so before being fired for cause. I suppose a lot of it would be that being fired for cause might be an issue in future employment, although in this case I can't imagine that any future employer wouldn't find out about this incident, especially not for a C-suite position where they'll research his history. I've even filled in some employment applications where one of the categories was about resigning under pressure to do so, although I have no idea how anyone finds out given how most employers have policies that they don't disclose the reason for termination. Or don't say anything other than someone's time at the company and job titles.
I would think he's entitled to whatever accumulated vacation time he has.
I've been laid off before where I got extended medical benefits and severance. But at the very least I could honestly say that I wasn't terminated for cause and still got a fairly generous severance package.