Tochacha (mussar) is a halachic chiyuv to give. It isn’t optional al pi halacha to ignore.
If you saw a group of beis yaakov girls sitting down with a cheeseburger and soda every day for lunch at the window seat of your neighborhood McDonalds, your obligation to intervene would be the same as when a group of beis yaakov girls are walking around the neighborhood with their hemline above their knees.