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Not good to leave them alone if the affected person is our friend or relative since we can do something to help them by giving advice and if we just leave them because they are not really worth the help then maybe they might experience more the worst. So its good to give them advice so they can realize that everything they do is not good and they should not bet excessively so that no funds intended for more important matter used for gambling. They should learn from their mistake so they can correct their bad behaviors.
Although we don't have obligation to do this thing but for sake of humanity maybe a simple word of advice can help them to determine what's good and bad for them.
I'm sure the thing about word of encouragement is truly helps those who are in need, especially relatives or friends. But bear in mind, that we also must set boundaries for ourselves. Helping or handling addiction cases is not an easy job. So if the condition affects in personal manner, we should just direct them to the professional. So in the context we can't help further, it should be okay to leave it as it is, instead the problem is also affecting us.