Oh my!  This is one of those times when you wish you could turn back the clock to prevent something from happening.  Why do kids do this?  (I guess I should be able to answer this, as I was one to cut a hunk out of my bangs - the night before school pictures!!!!) 

I guess you will probably just need to cut what is necessary to make her hair look as reasonable as possible, then work on growing it out again.  That is assuming you think she can handle long hair again.

Good luck - I hope this will work out.

Kids do the craziest things, don't they?!  My daughter took a hunk out of the front of her hair some 25 years ago.  She had long blond curls and was modeling at the time.  We had to cut her beautiful hair and it was a couple of years before she got another callback.  Maybe she just wanted to be a kid instead of little miss perfect model.  I found a photo of her the other day that reminded me of the time she cut her hair.  Now she is a PHD and teaches at a university.  She has two kids of her own, who will cut their hair at some point too.  Take a picture of her for memory's sake and know that it will grow back.

Once upon a time I got gum in my hair and I cut it out-a huge hunk above my right ear.  I was a champion bubble blower at the time and one of my bubbles got away from me and caught in my hair.
I have never cut my own hair since (I think I was 8 or 9), I even feel uncomfortable cutting my own spit ends.  I had to have a pixie cut to even everything out-I thought I looked like a boy!