If the neighbor’s kid is doing drugs, then going over to him to tell him that drugs are assur, illegal and damaging to his body will not get him to stop doing drugs. It probably will make him more resentful and feel more isolated and more likely to feed his habit. So you are not saving him, but actually hurting him.
Telling his parents, who probably are very well aware of the problem, will only increase their pain and shame and and their anger towards the interfering do-gooder.
Same with tznius- yelling at someone that they are not dressed tzniusly, lecturing or forcing rules down their throats will not change anything. It will only re-inforce the negative behavior, and then, perhaps, you too are guilty of being machshil someone.
These are delicate issues that must be dealt with chochma, by capable people who can bring about change and not by every busybody kanai with an agenda.