Nothing special is needed. Shoot the shot with her in the dress showing in the mirror (being sure to focus on the form in the mirror). Now put white paper over the glass surface of the mirror. Shoot the shot of her with the gown in front of her (with her in jeans). Use the same light for both shots. Now bring both images into PS and drag the Jeans shot with masked mirror on top of the first shot. Select the white paper using the magic wand. Press Del. The background photo will now show through the hole that Del has created. You'll now need to adjust the lower layer's size to include as much of the girl and gown as you want to show (you may have to unlock the lower layer depending on your workflow).
Here are a couple of similar examples (all of model Haley):
And here is an example of one of my Rockwell tributes:
Re: Need advice on a shot request
Note: You wouldn't want to shoot her in the gown from behind the mirror with the glass removed, because then she'd be reversed in the final photo. You want the final photo to look like she's looking in a mirror but seeing the other reality.