Writer,
True screaming and the like will not accomplish anything. But if someone witnesses or is aware of another Yid doing an aveira, be it tzinius, be it kashrus, or be it chillul shabbos, he has a halachic obligation to inform the violator that they are sinning and should stop. (The only exception would be if the witness knows with 100% certainty that the aveira-doer will refuse to heed his words.)