Yes you can shorten it. You will either have to regrind your recoil pad, get a slip over pad, or a different aftermarket grind to fit pad. Shortening the stock will make the stock a little bit shorter from heel to toe, also, sometimes a little bit narrower, at the point the pad goes on. Get her a slip over Sims Vibration Lab pad, or a grind to fit Sims or HiViz Xcoil pad. If you have never put one on, you probably should have a gunsmith fit a grind to fit pad. Even with the little recoil a .243 has, either of those three will make shooting it a more pleasant experience for her.
You can get a Sims slip over pad from Wal Mart for about $20. I put two on my shotguns (3.5" 12ga. and 10ga.) and they made a world of difference in felt recoil.