Teenagers believe they get freedom when they start making their own money as you can't control them by telling them how to spend the money. Your nephew should be watched because he might be heading down a path that might be dangerous and will need your help.
Having a helicopter parent is not conducive for teenagers. And looking at the scenario, the boy may not be what the mom thinks he is, since OP as a gambler examined the boy and thinks he is a responsible player, I don't think the boy is getting deceived. It'll be nice if OP spends time discussing gambling with the boy or watch the boy play games. Then, he'll be able to understand what exactly could be the boy's problem if there is any, and he'll make the mother aware of how to treat her son. A teenager can get into trouble if they've got no gambling guidance from somebody who is close to them.