i think if it was clear that what happened was not part of the way she normally dresses the best solution is to find a woman who can point it out discreetly. failing that , writing a note on a piece of paper that says your shirt needs to be adjusted or something similar, could be done. you could write the note, say excuse me , please read this. and walk away. she would be grateful.
I as a woman have several times alerted other women in this manner. i would never dream of telling aman that his zipper was open. but if this lady was clearly unaware how revealing her clothes were i’d say inform her as i said above.