Everyone should understand that gambling is good only in moderation, excessive enthusiasm for gambling leads to disastrous consequences. If you want to teach someone how bad gambling can lead to - show him examples of players who have lost everything in life because of the game.
If you want to tell anyone about the gambling, then you should tell do justice and tell both sides of the gambling, Good and Bad, advantages and disadvantages. We should tell both aspects and leave it to the person to decide whether gambling is good or bad for him.
We should not force anyone to join gambling.
Although in all conscience they have been knew a peril that will accept by him when decide to gamble. I haven't found someone especially in this forum who give a counsel that gambling is a place to get a lot of profit if he join in. Most of them just give a counsel about gambling is negative way that will he feel when become an addict. A good person will not force anyone to do gamble although he is an addict.