Greetings <S>

I assume you are not a parent?
Let me give you some advice as I have raised many children, have lived a long life (so far) and seen a lot of things along the way.
There is a lot of truth to the old saying "You can lead a horse to water but you cant make it drink". The horse will not drink until it realizes it is thirsty. This is the case with humans as well. You have this basic delimma before you.
Do I not say something and just let them go or do I say something to try to make them aware? What ever you say will not have any effect unless they are thirsty for what you are saying. But.... if you dont at least try to communicate your concern then if things do go bad you may feel pretty bad about not even saying anything.
I have been in this situation many times as I have raised many Teens and still have some on the way. I would at least say something. I would not try to drive it home but at least fulfill my obligation as their friend, their Father, their Brother, etc... and at least bring it up. But when all is said and done I would not work at convincing them but just let them make the call and go with it. I would also try to avoid the "I told ya so" after it blows up. The best thing you can do is let them LEARN from their own choices. After all the real primary function of a parent is to teach their children how to make better choices.
Regards,
KayBay