The victim can sue them and if the victim win, for sure the alleged Casino can't get away from it. I still hope they can solve it in a peaceful manner and for the victim, I hope they get justice for this. Though, I think this casino has a valid leeway to hold or forfeit the winnings like breaching TOS for example.
That is why if you're going to deposit and play with a big amount of money be sure to read the TOS of the casino and take into your heart these terms and follow it by heart, so you always have a good defense in case of a worse scenario, casinos are always checking your betting history, your login, and your behavior while you are playing, so be sure you are fully aware of what's happening if you are breaking the rules and still within the scope of the rules.