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Think of you the owner and you the employee as 2 separate people. Same thing applies for your co-founder.

If you cease to work as an employee it has no bearing on vested shares held by "you the owner".

If your co-founder is considering leaving (as an employee) the share owning entities (which include "you the owner" and "co-founder the owner") need to decide how much (money from share sales or actual shares) they are willing to part with as added compensation so as to keep your co-founder on and protect the value of the remaining shares they hold.

Increased compensation to your co-founder as a employee should come proportionally from all shareholders (as it would in a dilution) as it is a negotiation between an employee and the company as a whole.

It is best not engage in a deal whereby employees extract compensation from specific shareholders, as, however well intentioned that starts out, it can too easily become a pattern of extortion (or a means to seize ownership).