Of course, resisting and counter arguments is an indication that your remedy is getting to them and they're struggling with convincing themselves that what you're offering them is better than what they already have(Addiction).
 
For me personally, I'll not tell the client that addiction is wrong or bad for him, it would only make him defensive. I'll go straight away and talk about their hobbies and another talent they have aside gambling, help him see how much money he can make from dedicating more time to improving himself in the field and I'll use the reward system to encourage any improvement he makes in the process. Even the rebels are subject to rewards and can be agreeable when promised one.

When he's fully engrossed in his new hobby and possibly making good money from it, we would discuss his involvements in gambling and have an agreement on putting up a good gambling behavior(that's if he's still interested in gambling) which would be strictly monitored for compliance until I'm sure his main focus is in his job.

I think his healing would last long because the new profession wasn't enforced, instead it was crafted from what appeals to him and it's his choice, so he'll put in maximum energy into it.

I hope I make a very good mental doctor.