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.