There's absolutely nothing wrong about being a casino worker and also delving in gambling with the casino but while doing so it's appropriate to stay responsible with how the individual would wanna gamble. Your friend gambling away his salary is a show of addiction and he needs to stop it or resign from working there at least that may help him stop gambling.
He will probably need to learn to control his emotions, resigning wouldn't help but will only make things worse because if he has a slight addiction, he will struggle without a job because right now, he gambles after earning money, but if he keeps gambling without having a stable source of income, he will suffer even more. So his best bet is to try and control his emotions, gamble only with a limited amount, and stop when it runs out.
I also think that a casino shouldn't allow their employees to gamble only to prevent them from getting addicted to gambling and causing trouble for the house directly or indirectly, but I guess they would never do that since they would surely want the money to stay with the house which means if an employee is getting his salary and then losing it back to the house, they are getting their money back, lol.