The juice has to be worth the squeeze. No sense in fighting against fiduciary duty, minority shareholder oppression, etc., etc. unless there is some sort of value there. This usually means a successful exit before taking action.

I think we're saying the same thing --- that none of this matters, just walk away with the vested shares and be a friend to the company. Diluting his founder shares in subsequent rounds is going to be a nonevent, and diluting him to zero in an acquisition --- unless it's a seller's market or a bidding war --- may be as well. It's just not worth worrying about; I think the only real question here might be "do I take a buyout if offered", and this person is nowhere near that yet.