In my experience, even the person knows you and you know him very much and he trusts you, if he's not willing to ease his gambling addiction, there is nothing you can do. Especially if that person did not come to you to ask for advice, it just doesn't work, no matter how much advice you give them.
I'm speaking to my experience because I had a friend whom I'm close to, but now he's estranged due to the overwhelming debts he incurred, and he cannot pay it anymore. I gave him advice, although he didn't ask for it, and it just did not work.
Well, addiction doesn't respond to logic, it just deepens over time and creates an isolation sphere where that person is kept alone. When you are advising them, it can reach their ears. I know it's very hard to see your close one like this, and you have the exact solution, but you can't do anything. If you force it, you will break the relationship, and if not, your friend will be slowly poisoned. In that case, I would say stick by your friend and don't give any type of advice, support him. When you find a moment where he gets tired of the losing streak or anything related to it, that's the time to hit the right spot and advise him slowly over time. I hope that he will pay attention to your words, and when alone, he will think about it.