Without spending too much time on this, I thought of the same as well and knowing that:

Her dress, which was very unique had never been worn by her, because she bought it the day before

...should narrow down the events that this person wanted to attend. Here are the events that happened that day:

http://www.austinchronicle.com/calendar/2013-09-05/

...which, to me, only had one really unique event that evening:

http://www.facebook.com/events/1405498132998020

...which, possibly, would include a picture of that person either in the same outfit or include a profile picture of that person commenting or liking the event.

But this is just a guess...