This guy hasn't caused any trouble to his friends but it's a worrying situation to see him waste most of his salary every month... -snip-
You are right but such a situation will destroy his life and his friends slowly. I'm guessing your friend isn't married because you haven't told him about his family at all. But I can say that over time he will become more and more broken and it will impact his relationships with his close relatives and people around him.
I'm worried that he has started trying to get into debt to gamble. You may not mention it here because he hasn't done that but one of the bad effects of gambling addiction is that you will become addicted to debt. What would you do if he tried to owe you money?
Tricky question but it should be a no, and if he insisted or threat you like your friendship is on the line, then things would be wrong on a different level. Your task as a friend, although it is not required, is to guide him/her to better life decisions but still, it depends on the individual whether he would follow your suggestions and advices 'coz we cannot for them anyway. Even if he becomes mad, let him be than to be a part of his potential addiction. And if that's the case, try consulting his family about it and convince them to have your friend be checked by professionals for the betterment of his situation.
But going back to the main question, will he really stop if you won't gamble? 'coz if he's really into gambling, he would still do so Your act of stopping, will just lessen the engagement but won't guarantee that his friend would do the same thing on times he's just alone. If he's already at that point that he goes that much with his bets, then he's basically problematic of it. Try talking to him in private and make him aware of things if he's not yet. But again, it will depend on him.