Just let them continue gambling until they stop on their own without any instruction or advice, especially if they have a family. It will be a devastating blow to see their family suffer as a result of their poor decisions, assuming they can turn things around on their own.
Reminding someone like an addict is very difficult. When their balance is low or empty, they might stop, but when they have more funds, they will inevitably continue because, in their minds, that's all that comes to mind, just like when we open social media and watch a football game, football is always the first thing on the page.
Imo gambling addiction can not be stopped itself. If you let person gamble non-stop, let him continue gambling when that person has addiction, that gambler will never stop. People always find way to find money. If a person has a goal, he will do everything to reach it. In fact, family sometimes are the reason why a person can not cure from addiction, because they are always kind to any member of family and can not take radical decisions. Family prefer cure process be slow, painless, merry. I have see times when parents, knowing their child is addicted to gambling, after seeing how he suffers, give him little money for play...