I'm in a similar situation, as I also have scoliosis Cycling has so far been the lowest impact exercise I've found. I have a Trek 1000C that has a suspension seatpost. It does a decent job of cushioning the impacts, so a suspension seatpost might help. A full suspension MTB with slicks would also help. I don't have one of these but I have been considering one. A little training might do a lot of good, too. I would try to get her to start anticipating the bumps & holes, and lift herself slightly off the saddle just prior to the impact. I do that and it helps. Also try lower air pressure in the tires.
Good luck.