Anyone that is already at this worst state, expect that he's not anymore at his good and critical thinking. He needs a medical practitioner that will guide and help him throughout the process, someone that he can trust and feel assured that he'll get recover soon.
Acceptance is the key. Being in denial won't help these gambling addicts. It will only make their condition worst, until they don't have chances to recover and feel good anymore.
A person will start thinking about changing only in some extreme cases, when he achieves something bad. I noticed that most often it is some kind of illness, some of them are more noticeable, some are more difficult to detect, but the person will change and those around him should notice it first. Depending on gambling, these can be nervous breakdowns, or frequent mood swings, because a person cannot behave the same way when he has big problems.