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.
Well, in my place someone who loses big in gambling already abandon their family instead of trying to fix the situation. So expecting them to stop on their own is almost impossible. Like you said, once they get more money, they often just go back to gambling. That’s why it’s better to help them reduce their addiction rather than wait for them to quit completely. Teaching them to use smaller amounts of money instead of betting big all at once might be a better approach.