SaysMe -“Speaking loshon hora also affects people, and we cannot say which is the worse aveira. Can you expect ppl to always be able to stop themselves and never fall?”
No, people do Aveiros, but they should expect others to call them out about them. I’ve learnt the Chofetz Chaim and I can only say that the girl who told over what the lady said is Oiver the Issur of Rechilous. The lady herself, probably did nothing wrong, because the OP admits she didn’t dress Tzinus and she has a problem with Tzinus (in other words -it ain’t her first time).
So the C.C. says you can speak LH in this case!