Several comments:
Men should not be discussing this issue especially online. Obviously you have been looking where you should not. Would you tell your wife about this scenario? I don’t think she would like this in any shape or form.
As a woman, I have difficulty approaching other woman who are wearing less than tzisudik clothing, so it certainly would be inappropriate for a man to do so. However, in the case of where the button popped and the woman was totally unaware, asking snother woman to inform her would be the most appropriate. Handing her a note will let her know that a man saw something he should not have and cause her great embarrassment. In the case of the tight skirt, most women are aware if their clothing is tight, so a man looking is inappropriate and he should not comment.