Since the casino is offline, it means it is registered in the country of location and operating under regulations. The right approach in this case is for the winning to contact the authorities who will not delay to intervene in this case. Where this fail, I think a legal redress will be a suitable option to use to get the money. As long as there is no violation of any forms, the money is not supposed to be lost.
Yes, it's easier to sue them and just go with the agency of the government that handles the games and gambling. And they might have some fear about any complain that will be brought to them. If they're too big to be complained, just pile it up and the authorities will come to solve them if they often receive complains that are the same from the past that they've taken. As long as there's a noise, they'll start to do a better service if they don't want a bombardment of complaints.