Imo curtailing gambling addiction rests on the gambler, it's a personal decision they must take on their own, most times maybe after a huge loss, it could just lead to their self realization that if they don't slow down they may lose something more valuable in the future.
I'm not sure that only giving advice could help. Gambling addiction is in medical way equal to.any other addiction and such person needs professional help. Friends and family can be a support but in most cases that will not be enough, unless maybe in some early stage. Also, similar like an addict, family very rare recgonizes addiction symptoms until it's too late.
Big loss of money might be powerful trigger to admit the problem and addiction, however for some even that isn't a wake up call.