Wendy, I feel your pain. The only thing I could suggest is to use a sewing machine to make a wide row of zig-zag stitch at the length she would like (you will probably need to cover both sides of the scarf with tissue paper so that the yarn does not become tangled up in your machine). You should probably do the same thing to the other end, just so they match. Then cut the excess away, but not TOO close to the zig-zag. Tear the tissue paper away. Then I would take some of the same yarn (maybe you could salvage what you cut off) and make a row of single crochet over the zig-zag to hide it, and then put your fringe back on. I know it's tedious, but still less work than making the whole thing over again.

Did you crochet back and forth or did you start each row at the same end? I chrocheted a jacket one time and had it too wide across the shoulders. I was able to pick it back, one stitch at a time, by cutting the yarn at the end, but it was very time consuming, but it did work. If you started each row from the same end you should be able to remove the fringe and rip it back easily. Then just replace the fringe at the length she wishes.
ivamae