my guess is that his trades weren't executing at the price he thought they were. often times, what you see on the order book at the spread and what actually executes the millisecond you submit an order are two different things. that's the danger of the market order function, especially in a high slippage environment.
a better way to get market order functionality without taking that risk:
-use a limit order instead
-go to advanced > execution = "allow taker"
this way, the order will never execute past the limit price you set = no unexpected slippage.