The main thing that the management of the casino will do immediately is they will first carefully investigate their users to see if they have not violated any policies and rules in their gambling.
That's the first thing that they'll do before they release the money to the gambler and that will come into a long investigation if they suspect that the user won that by surprise. They're always suspecting their winners if that's big money to be withdrawn.
The battle will be long into the courts if the winner don't have a lot of money to pay for all of the cost of those trials and the casino might have just to settle it for a lower amount but it's up to the petitioner/gambler if he'll agree on that deal.