If we would look back at the begining of this post, we would find that it is untzniyusdik for men to comment on women’s tzniyus. By describing something, they imprint it further into their minds, from being short term memory (and forgetting it) to long term (not forgetting it). It is true- communicating in any form what you see/hear makes a bigger impact on what you remember (it is a good study tip- repeat back what you learn!) If a man sees someone who he knows is dressing untzniyusly- tell your wife to tell her! Speak to the school (if it has to deal with school-aged girls) so that the Morah (WOMAN- not the moreh) can speak to the class in general about it! Women are able to say things in a more gentler, easier to handle way that other women can accept. Men say things in a way for men and women say things that are in a way for women. That is the way Hashem made us- men and women are referred to differently in the Torah (Vayomer vs. Viyidaber).
While Tzniyus here is generally refered to as regarding clothing, in essence it includes more then that. For example- speech and mannerisms which MEN have to work on also! It may not be “attracting men’s attention” as the mantra goes for women, but it is still “attracting attention” and is just a chillul Hashem! It is just easier to comment on another’s issue that you dont have to worry about. MEN have just as much issues with speech and mannerisms- I see men on buses and in the street yelling, talking loudly on the phone, acting rude and unmench-like, and overall acting inappropriate for a Ben Torah. A Man can set the tone for teaching others and should work on themselves, especially if they are going to correct others.
(By the way- just because I said these things doesnt mean that I am untzniyusdik in nature or dont dress properly. I do- but I wont describe my habits as I dont feel the male readership should read how women should/should not/ do dress. )