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.