ohr chodosh: One is obligated to give mussar when they feel the person will listen. Additionally if one does give mussar, it needs to come from a place of love for the mitzva and for their fellow Jew, not from a place of contempt or disgust of the other person.
You should not give mussar in a circumstance where you risk pushing the other person further, which unfortunately is frequently the case when it comes to tzniut.