In this panic, the price of bitcoin drops as Diana dumps her 1k BTC market sell on the book. The USD comes from whomever purchases - there is no liquidity problem.
Why does he need to call you back? Stock market crashes are nothing new. Bitcoins are basically the same thing as stocks in terms of the way they're traded.
No one is saying a price crash isn't possible. But it won't be caused by some liquidity issue, it'll just be people selling off.