Yes, I agree that is a lack of self-control. Turning $20 into $700 will be a dream for many people so when you reach that, you should stop gambling. There is no big chance to win more instead of losing the money you got. So it is better for you to stop gambling and enjoy your winnings.
But some people will be greedy and want to chase more wins. Instead of winning more in the next rounds, she just loses money and even everything. She has her lesson so we hope she doesn't make the same mistake. It is better to enjoy the win while we can. After all, we can play gambling on other days.
Turning $20 into $700 is the biggest achievement in my view which is not easy for peoples during gambling because this need good luck and some solid workout but this also increased greediness because from here always real test started.
Gamblers with good experience always try to cash out and enjoy this amount because they understand consequences of staying into gamble, but newbies usually want to go ahead which always wrong decision and this give them shocking end which always regrettable for them.
In early days of my gambling I have done few times and able to have withdrawal just once because of my greediness but once someone give me tip then I started real enjoy and never go ahead while I am having wins in my side because this is always good for anyone.