He likely did a credit spread and one of the options exercised early. Robinhood shows that you owe $x, but if you look closer you have another option that guarantees that you'll get the stock to cover it, you just have to email robinhood support and tell them to exercise that option too. Very sad that he likely didn't even owe that $700k, just misunderstood the UI in his panic.

https://www.reddit.com/r/RobinHood/comments/9guf3n/doh_some_...