Discuss with her: why did she pick it? Can she achieve the look she wants with another dress?
I suppose she has a desire to look more mature, and honestly, severe will not necessarily translate into ugly. If it's a long black dress, then maybe sleek updo, and a big fluffy white flower. Contrast textures, add a pop of color and make her stand out in a grown-up way.
Ultimately, I believe you should buy her the dress she wants. So if a little pressure does not work, let it go.