Do we have a moral obligation to "Fashion Don'ts"?

So I just waited 10 minutes in line behind a woman at Walgreen's who was paying by check. Argh! Who knew people still did this?

And on the way to Walgreen's, I saw a woman in a lovely white summer dress. She was also wearing light blue underwear that was creeping up her butt. Did I mention the dress was see-through? In a situation like this, should I have told her that her dress was completely see-through? On the one hand, I think I would like to know if it were me. On the other hand, I would never let myself leave the house like that in the first place. Does she not have a mirror/roommate/boyfriend/husband? What is the proper protocol? It's not like she could have done anything about it unless she had a back-up outfit in the office. Or maybe she had a jacket she could have wrapped around her waist. I think if it's something like a fly being unzipped, you tell the person so they can fix it and save them further embarrassment. A see-through dress? Maybe I would have just made her embarrassed the rest of the day. Or maybe I would have saved her from making the same mistake another day. Dilemma.