For a more permanent Back stop, cut carpet strips about 1' wide by however long you want your target, stack up to desired height and compress with all thread. You should be able to get the carpet free from local remodels, and with a little effort, end up with a back stop that lasts for a long, long, time.
Another one I saw, but never tried, was a Burlap sack stuffed loosely with sweatshirts.....Stuff looser for low poundage, tighter for high poundage. hang it with a little swing action to absorb energy.
Cardboard works as well, but the above two might last a little longer if she's shooting a bunch!