You can ask them if they are ok with their current situation.  Then the next thing depends on their answer.  If they say they are ok, it means they don't need anyone meddling with their affair or gambling activities.  But once they opened up about their regret on gambling too much, then that is the sign to give them some advice.

I think the best for them is to know financial management down to bankroll management.  Explain to them the importance of knowing these two.  If you may, you can ask a question about the signs of gambling addiction but not directly insinuating that they are already addicted to gambling, but tell them to have self-reflection.  You can give the link of the questionnaire and let them answer it in private, and let them realize where they are from being addicted to gambling.