In my opinion you have to be extremely careful how you rebuke someone, especially on the matter of tznius as it is quite a touchy subject especially around people who don’t keep the halachos to the fullest. Tznius is a hard mitzvah for women so before you are quick to judge look behind and see the real story. Once you know the background of the people, only then can you judge and rebuke.
Make sure your tznius is up to scratch as well before you do anything otherwise you will be dubbed a hypocrite.
Also be tactful in how you approach the subject. the word tznius switches lots of people off and they dont listen.
BeHatslacha.