I have been thinking of the same question lately as I have a friend who is doing stuff without her parents' knowledge when she already is in a very complicated and delicate situation.
I tried to talk to her a few times but she got agressive and annoyed at me leaving me hurt that hey I am not the one getting into trouble here but just trying to help her out and she wont even realize my intentions. So I was annoyed for a few days thinking that is this worth it, spending time with her acting like her shrink.
I think grown up ppl are capable to see the difference between the good n bad and even when their good friends are clarifying it for them and they wont listen, theyn y bother wasting time and energy on such ppl?
On the other hans it is also difficult to let such ppl contiue with their outburts as you know the consequences and they might not be clearminded to see what might happen to them....its a difficult question