Can she “get in” it? No? Forget it, unless it needs to be hemmed more than 5”. If it does, there’s a chance a skilled seamstress can use that left-over hem to make it big enough, especially if it’s 2 layers (a sheer over an opaque, for example).
If she CAN get into it but it won’t zip all the way, please don’t try to force the zipper up! If you do, it may break the zipper or stress the fabric around the zipper so much that it’s unrepairable . Chances are that it will zip part way, at the bottom. At the top of the zipper determine how much space there is between both sides of the zipper. If it’s less than 5”, it can very probably be let out enough to fit her, especially if it also needs to be hemmed & there’s some extra fabric from that hem.
Don’t worry, unless you could fit another girl in it with her. Again, a skilled seamstress will be able to take it in & size it down. *There’s something about it that makes it not perfect – the straps are too wide, she’d like the skirt to be narrower, it’s strapless & she wants straps, there’s too much froo-froo, not enough froo-froo…you get the picture- I personally know a Skilled Seamstress who has a lot of experience with “re-styling” and has been quite successful making other Princesses happy!
Chances are pretty good that she’s going to need alterations. A Simple Hem is about $30 and other alterations can run as high as $100, but your seamstress will advise you of that & work with you.
This advice is not strictly for Prom, so save it for other formal events that will come up during the school year; Homecoming, Spring Fling, Fall Fling, Winter Fling, etc. After the Big Event, consider passing the dress on to someone who can use it. Our planet isn’t getting any bigger & it behooves us all to recycle whenever we can.