However if you spend directly bitcoin ( please correct me if I am wrong) you don't need to pay any tax on this... .
That's wrong. From the moment he earns money in the casino, a taxable event is produced for which he will have to pay taxes, unless where they live they do not tax casino winnings for taxes. And when he spends it, a taxable event is also created on the difference in price when he won it and when he spent it.
Is this applicable in crypto casino? I’m curious on how the government will knew that you win on a crypto casino.
I think this the reason why US doesn’t allowed operators with Curacao license to operate in their soil since they can’t tax US players. But in general, All gambling profit should be filed for taxes. Luckily in my country, it’s up to you whether you will declare your casino profit or not since our IRS counterpart tax agency is not strict when it comes to tax implementation on casino profit.