Delate all accounts on gambling sites
Even though you suggestion is short,but for me,this is the good advice for those who wan to quit gambling.Maybe there are a lot of work to do for deleting our account on all gambling site that we already involve because we need to contact their admin for deleting our account on their site.But this method is good.
He can always create another one but let's just say that he has the discipline to not create another account in any gambling site, there's still a tendency that he'll gamble in real life. Gambling is just everywhere. Maybe right now he's not gambling but when he visits a place that has a celebration, I'm sure that there are people gambling inside. And since he's known as a gambler, people would encourage him to play with them and it would be really hard to say no to those people because they are his friends.