Given that he quit just in six months, probably he did not even wait for his RSU grants to mature (1 year cliff for stock grants). The situation might be much worse that what appears.
It's very likely he had to forfeit everything, including any signing bonus (1 year timebomb is standard).
This is what having fuck-you-money gets you: The ability to walk away not care about the financial consequences.
People are funny. Millionaires get billionaire-envy and are sometimes willing to risk jail to add the extra zeros.