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.
Some of these victims to gambling addiction can not be able to stop on their own again, they need help via a very strict method before they can be able to stop and some persons doesn't even get to stop no matter the force that is applied to help them. Don't you see some parents that steal their kids savings and use it for gambling, that's to show how addiction has eaten deep into them that they don't even care about the wellbeing of their family.