IANAL and a lot depends on the type of company, where you incorporated (Delaware C Corp?), and whether you have any type of shareholder agreement. (This scenario is the #1 reason to have a shareholder agreement.) That said, you presently own vested shares in the company, and that means you [likely] have minority shareholder rights. If the company does not act in your best interests as a shareholder, then you have cau…
Just pointing out that there are other reasons to leave aside from rational economic thought about this individual's expected ability to produce higher output elsewhere. They may dislike the team, pivoted idea, or leadership decisions. But they may also want to keep hold of the equity in case it gets big (or they can sell it or a liquidity event occurs).