Etiquette - Should I have told her?

This morning I got on the Overground at Stratford and sat down, I then looked up and saw the lady opposite (well put together, and sitting with her friend chatting), was sitting with one leg cocked up over her suitcase, she was wearing a short shirt and a pair of see through knickers. I know this because you could see everything, down to the fact that she was fully shaven/waxed down there, her legs were wide open. I didn't know where to look. I contemplated writing a message on my phone and facing it towards her but the train got quite full. A lady sat next to me, looked up, saw what I saw, looked at me wide eyed and moved seats. A man then sat next to me, looked up wide eyed and then hid in a book.

I usually let people know if their bag is open or they have dropped something, but I just didn't know what to do in this situation.

Should I have told her?

EDIT- I am also female