The best time was when he still gambling with his normal senses but overdoing it. If you miss that time or he didn't listen, it becomes difficult for him to listen when he has become an addict because by then it's beyond his control.
I do feel for addicted gamblers especially, when you're trying to talk to them and they turn deaf ears on you. It's a sign that they wouldn't give up on gambling so easy. If only they can quit.
I think that friends, long before their addiction to gambling, can show their character in one way or another, indicating that a person can be very prone to gambling. Knowing a friend's temperament, you can always remind him of where his gambling addiction can lead him, even at the initial steps (games) of such a person.
In any case, funds are always needed to play, and if you see your friend starting to lose control of his finances, talking about his addiction shouldn't wait in the wings. Show him all his mistakes, as well as examples of those who have become who, having a similar passion for games without stopping.
Most of the time, the examples will not be positive, and the prospect of becoming the next victim of gambling addiction can often stop and make your friend think.