First off, you need to find out her motives. Are you sure she's doing it just to be popular, or did you just assume?
If you are correct, then here's what you do. Tell her your concerns about this. Maybe you're worried about the pain she'll have to go through, or that she thinks she has to change herself to be cool. Whatever it is, make sure you phrase your sentences to show your concerns, not that you think it's a bad decision.
If she still wants to pierce her nose, you just have to let it go. This girl is still young and has plenty of time to learn that she doesn't need to change herself for others. Although, you should help her find a good place to do it. The least you can do is prevent her from getting infections.
Good Luck!
Nosey"