i also think that forcing someone not to gamble is the same as making them even angrier and will actually fight us. the approach taken should be with understanding and empathy, where we should be able to advise them face to face. if they still resist in the process, give him more advice and don't allow him to gamble more than he can afford. because stones that are dripped with water will also change shape, like people who are addicted, if we continue to give advice and empathize with them, maybe it will help them to soften and reduce the intensity of their gambling.
Yes, change is achievable, and the approach you both mentioned is the right option for healing an addicted gambler. As he'd need to discuss his troubles with us like we are friends, forcing him or yelling at him only causes a countervailing reaction on the player which can chase the gambler from our vicinity or presence, not wanting to have a conversation with us, hence making him feel very comfortable around us is the best healing technique on the addict.
In addition, tracing down on his behavior as a child also matters, many therapists, tend to discuss the person's past and figure out ways to get him out of addiction.
We all love our pasts, that's why we are happy whenever we get in touch with a childhood friend. Therapists can tap into this trick, as happiness is the goal, since the gambler has been sad and in pain during his period of addiction. Keeping him happy will drag him closer to us, and he'd never feel any need to avoid us, once that has been achieved, the player will be one step closer to being addiction free.