The saddle is going to be wrong and the old saddle might not have been an issue it could have been poor adjustment for the rider. However if the position of the saddle on the seatpost is moving like others have said that is a clamp issue and that may require a new seatpost of higher quality or taking apart and reassembling the one you have currently and using a mild threadlocker or proper grease and torque.

A good saddle should be supportive of your sit bones and should be the proper width for them. Too wide or narrow and they aren't properly supported and too wide means you could easily chafe your legs or have to move forward on the saddle. The padding can be there but too soft and "cushy" and you are compressing padding till you get support which creates pressure points and that creates pain. However number one goal should be to make sure the old saddle is properly set up for her. Make sure it is adjusted fore and aft, the angle is right and the height is right and she rides it for a decent bit to confirm. That is always important and frequently over looked.