Everything boils down to emotion and greed some people do not know how to control their emotion and they don't also know how to control their greed. When they are in for it, there is absolutely nothing you can say unless they exhaust all the money on them before they will listen to you.
There is nothing wrong with gambling if it is done like a fun and not when the gambler uses all he has to do bet a particular game and which eventually when the outcome does not favour the gambler he will loose everything and plunge into emotional damage.
When someone is on that state they will not hear a word of what you are saying, they are so focused on their goal of trying to make money when they gamble that any comment which has to do with how this is a bad idea will get immediately ignored.
And what is even worse is that once they are out of that state they are not going to remember either that you tried to stop them from this behavior, and they may even recriminate you about not doing what you could to stop them, and when I have being on the receiving end of those recriminations I simply leave as there is no point on talking with someone that is not being rational at all.
Emotions are very difficult to handle when you are in a middle of rage and sadness together as this is what produces when you lose money on gambling,this is more emphasized on games where you keep playing continuously and where you see the result instantly thus accumulating lost amounts.In such situations no matter what we try to tell them they will ignore us,even me personally if I was in such position I would ignore anyone talking to me because of this rage that heats a person up.The moment where should we talk to them is after a certain little time that they have quit their session,this I have seen to work out for me as I am more lucid and clear and I understand better what is being talked to me.