If you try and police what they wear to a nightclub, it is easy to end up with a situation where they leave the house in one outfit and arrive at the club in another.
If you are going to a more formal do where some twat will hold her 18yo fashion disaster against her for life then you need the "granddad acceptable" dress.
I used to march the girls via Zara, Mango and All Saints. Generally something all round acceptable was found.
In a few years you will be able to buy her dresses you both like, right now you might have to settle on a compromise dress and accept that she may only wear it once or twice. I'd throw a bit of time and effort into going shopping together rather than just trying to throw money at it. In ten years time she will remember the coffee you drank on the freezing cold bench, not the dress.