Every trade platform I've used provides two options: sell at current market price, or sell at a specific, unchanging price. So it sounds like he misunderstood the option he was selecting, or something.

That's how every trading platform works. You can put in a limit order where you buy or sell at a certain price, but it's not guaranteed to go through because there has to be someone else willing to take that price. You can also put in a market order, which will always go through, but the price will be whatever is available on the market at the time.

That reads like (s)he was sitting at the computer with the intention to sell at $350, and happened to be there to see it quickly drop to $275, got nervous and cashed out. The unlucky was that (s)he never would have thought to sell it except for seeing the price crash in real time.