but I afraid when I introduce gambling to someone who cant control himself or herself then it can become a problem, and I can become the one that make him/her become addicted
Then the best way to do I think is never advice someone doing such thing. I know that your intentions are good but the outcomes might not be good as well. So to avoid your conscience being bothered by the miserable possibilities then don't be the one who act first, let him approach you and seek for help. Everyone should understand that gambling is good only in moderation, excessive enthusiasm for gambling leads to disastrous consequences. If you want to teach someone how bad gambling can lead to - show him examples of players who have lost everything in life because of the game.
What a nice way for them to realize the bad effects of gambling, it's like presenting the life of a drug addict into a rebel teen who attemptts to try it too. Through this, adpiring gambler tend to become csreful becsuse I'm sure that no kne wants to be broke, righy? 
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