I am more interested to know how much of that payout he will be taxed on
But also, that's just federal taxes. California adds 13.3% on top of that (for any income over $1 million, which is most of the income here).
But also, this is a stock grant, which generally get taxed as ordinary income, so I would expect 37% federal, 2.35% medicare, 13.3% California tax, adding up to 52.65% in taxes.
The particular shares involved _could_ have used a different legal structure, but that's not my understanding based on the articles I've seen about this.
But even in your hypothetical it would not all be capital gains. It would be ordinary income for the value of the shares at the point of the grant, and capital gains for the appreciation since then.