It is very difficult to get a gambling addict out of gambling if he does not try and has no self-control.
It is difficult sometimes even if he wants to and he tries, so if he doesn't want to and it is a relative who tries to force him he has a bad prognosis.
In the case of someone who is just 18 years old, the legal age to gamble as mentioned in the thread, I think the best thing to do is to try to normalise it by informing them of the potential risks. If it's your child, it's better to go to the casino with them one day, and spend some time gambling in a controlled way but also explain to them how many people you've met who have had serious problems with gambling.
I think this is better because at the end of the day, being of age, he is going to do what he wants and if you try to convince him not to do it out of fear, what will happen is that he will do it on the sly.
Gambling addicts need some help and unfortunately, they cannot help themselves to get out of the cycle without the help of others. It will take time because an addict will always seek what he is accustomed of but he or she needs to endure so that sooner, he can pull himself or herself together and start living normally.
For the boy who just turned 18 years old, he might've been a responsible gambler but he is still in a risky position because at that age, he will still have some hardships weighing the situation which in turn make a bad decision in life. It's still to soon for him to take this path and it's best for him to find another hobby because he is still studying.