We use to use styrofoam we got from boat docks.I use to shot an 80 pound Spoiler in it. Another good backstop are the ones made from canvas and filled with cotton or old rags as long as field points are used. I think they are called Dead Stops. With out knowing the poundage of her bow, its hard to say but there are lots of options. Even an old bed mattress maybe.

Stay away from negative people, they have a problem for every solution.