I loved shopping for my prom dress. I tried on EVERYTHING (including a $3,000 Nicole Miller that for some odd reason just didn't fit into my budget...hmmmm!) Tell her to try on things, even if they may not seem too great on the hanger. I actually wound up trying on a turqoise sequined gown on a dare, and fell in love with it (the price didn't fall in love with me, however).
The dress I eventually found was a stunning Black gown with a train (which my mother sewed into a small bunch in the back-really hard to explain!) it was a deep v-neck, and the back went into a very deep V plunge, above the train was an O shape cut out. The straps were rhinestones. It also had a split up the right side to the knee.
It was not at ALL what I had in mind, and I didn't think it was too great on the hanger, but once I put it on, I did not want to take it off!
Just tell her to be openminded and try on lots of stuff! You'll both have a great time!