Ok, suddenly the casino you're playing asks you for proof of income to be able to withdraw your winnings and to keep your account in good standing, your bankroll every month is $500 and you provide proof of salary that you are making $500 net every month, will they accept that proof will they accept that you are putting all your salary for gambling, will this violate the principle of gamble for fun, because you are working to gamble based on your proof income.
I'm not sure about it but I don't think the gambling site will be that keen to review your expenses upon your issuance of Proof of Income. It's all a matter of self-principle. You yourself feel like you're doing too much and have begun to spend most if not all of your money in gambling. The casino could care less about where you put your money. They just need the proof of income to make sure you are not a robot or a hacker if I'm not mistaken. So as a for fun gambler, if you feel like you have already gone too far, so as to spend all your salary for gambling, maybe how the gambling site thinks of your expense report should be the least of your concern. You should have yourself checked to a therapist or a psychiatrist because that could be a defining sign you are addicted.
I would agree with the fact that the gambling company couldn't care less about where you spend your money, just where you got it from, thus the reason why they are asking about proof of income. And I also think the OP's question may be a bit deep-seated and could be an indicator that the person he is pertaining to is now at least subjected to gambling addiction, that's why he needs to be checked if ever. The casino wouldn't care if you spent 500 bucks or 500 thousand bucks on them, it's how you think about spending that money that would matter. if you think you are doing too much already, then maybe it's time to reconsider your choices and see if you can get help in a way or another.