The options were likely 10k when he was issued them at hiring. When leaving the company, he would need to purchase those options (likely within 90 days if it's a shitty policy). Then, the real kicker is that he would have to pay taxes on the on-paper gains between the 10k and the current valuation. So lets say the company was worth half of what it was at IPO, he would now own 2.5m of stock, owe taxes on 2.49m of income, and have to pay that off with early engineer salary and no liquidity on his equity.