Once a gambler breaks that taboo and they begin to borrow money to gamble there are very small possibilities of succeeding on making them see how mistaken they are to do this.
As it is obvious that at that time they have lost any kind of common sense and they are willing to do anything to fuel their addiction, this is also the stage in which even their loved ones and friends distance themselves from them, as once they owe them money, money they know they will never recover, they become unwilling to keep helping that person to destroy their life by financing their addiction.
That is not going to help the victim; in fact, I think that's when their loved ones should stay by their side to let them understand that what they are trying to fully involve themselves in would have a bad side effect in their future, unless their relatives are really feeling less concerned about the victim's well-being. Some of those gamblers who borrow money might not take those loans from their relatives but from other friends or fellow gamblers. Some of them, after taking those loans, don't bother to repay until it begins to cause issues; that's when the report will go to their parents or relatives, and they will be the ones to cover the debt if they have a good heart. If you see any of your relatives going down that road, it's up to you to start correcting the person.
The way of help that close relatives or friends might be different and like what you mentioned if you are caring with the person who got addicted you needed to find ways to let them realize what is happening, you also needed to spend time if you care more about the person, though we have different approach the effect of your concern is always valuable in terms of redirecting the persons mindsets, to the point that you already needed to ask or seek for professionals to guide you and the person itself on how to resolve the issue.