It's not that complicated.
Give her any old wasteland outfit and she will gush(paraphrasing)"A dress?!".."How did you know?!..aww thanks".
Her goal is not a dress.Her goal is the survival and viability of the Brotherhood.
A dress is her [censored] way of deflecting and is a red herring.
Not true.(About the dress.)
It needs to be Formal Wear, You can get it off Montimer in the Ultra-Luxe