It is definitely a problem when a teenager is gambling with money that is not even theirs, however most of the time this is just part of what being a teenager is about, as when we were that age we did all kind of crazy things that when we think about it the first thing that comes to our minds is what were we thinking? However an adult that is also in that situation is a completely different thing, as they are immersed in a very unhealthy dynamic with their family by not earning their own money at that age.
Teenagers are rebellious, they also are exposed to the worst peer pressure and always want to be like their friends. Maybe his friends might have been spending money they would have gotten as gifts from their parents or family members and the boy wanted to feel among.
He wanted to have his own money so he can spend like his friends or buy the latest cloths. Whenever teenagers starts gambling, they're not doing so because of entertainment but because they need money to spend and at this age of still been a teenager, they want freedom.
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 they made. Your nephew should be watched because he might be heading down a path that might be dangerous and will need your help.