Some stores offer the Specialized saddle measuring service: you sit on a pressure measuring device which shows the width of your sit-bones which are what you should actually be sitting on and they can then tell which width saddle "should" be most suitable.
Never tried it myself, but possibly some forumers have tried it - you might find out on the ladies section as suggested
Good luck - but remember that, even when she has a comfortable saddle, as other forumers have said, you will need to help her develop her fitness gradually. Regular short rides at an easy pace, padded shorts and make sure that the saddle height is right for her. Your LBS should be able to help if you're unsure.