I think the best time to make him understand when he lost something that time he will be free to understand your points briefly why Gambling is not a good thing. And there is not shortcut for success in life and definitely it is not gambling.
Although we are also a kind of gambler when we predict something but the difference is only we are not losing or gaining something. And we really enjoyed gambling but over the time it will become an addiction that has a great potential to make our life hell.
It is an addiction and there is no proper solution that can help to leave this addiction instantly as it takes some time . From my perspective if you are his/her friend then you should help that person to be busy in positive things and ask him question like why you are doing, what you will and after that what you will do etc many question and do helpless him to think about his future so that he can connect with his inner soul.
> Some gamblers respond well to antidepressants, narcotic antagonists and mood stabilizer medications. Oftentimes, a person with a gambling addiction also suffers from bipolar disorder, depression, ADHD or obsessive-compulsive disorder, so medication or therapy to treat those conditions can alleviate gambling addiction. Gamblers Anonymous and other self-help groups help many people as well. [source](https://www.psychguides.com/guides/how-to-help-someone-with-a-gambling-problem/)
This above quoted lines and link will help to get brief Psychological knowledge for gambling and there are many Yoga techniques that help him to make strong will power.
Thank you & Have a Good Day/Night :-)
Gambling is really bad. Yesterday I lost 11 steem and I have promised myself not to participate in gambling again. I can see that there are so many people engaged in gambling with their steem balance. Although some people are making money but most are loosing money. I do find that gambling is not ethical too. Because your win is someone's loss. So although it is as per the rule & transparent but morally it is not good. Because if someone is crying because he has lost and I am happy & celebrating because I have won but the fact is that my winning is at the cost of someone's pain. That is why we should not involve in betting and gambling our hard earned money. That is why I have decided whatever I have lost, I will forget and from today I have promised myself that I will not bet again or indulge in any kind of gambling activity.
I have also seen people, after loosing money they used to borrow and bet again and this continues and they get buried under the debt further. There is no end to it, as it is like an addiction and they suffer more and more and it affects their health as most of the time they are under stress and it makes the life pathetic too. So tell your friend not to gamble, rather do hard work and make the process come slowly and steadily. There is no alternative to handwork and once people understand this they remain aware of this. If possible also tell the failure story and how winning in gambling is just short term success and in the long term everyone will lose.
Just try to convince your friend (who are winning) to stop gambling and withdraw and those who are losing, you can convince them at least in a better way as they are losing continuously.
The best time is : after they suffer big losses. But make sure you talk about their future, dont make it worse, but stop and make it better.