You should ideally sit on the little sitting bones of your bum. If you want to be comfortable then those are the only two spots that should make contact with your saddle as after some time these little spots become feelingless like your elbow do (try pinching your elbows, no feeling as they have become feelingless from leaning on your elbows your whole life there deadening the nerves in this area). No knowing this, think of a soft saddle, they are incorrectly designed with the concept of spreading the weight over a larger area thereby reducing the pressure your bum and other parts feel. However, as your rightly say, there is movement and even worse blockage of veins all leading to pain. The softer the saddle the more you sink into it and the more of your bum that touches and can experience this pain, but not enough that it will deaden the nerves in the whole area. Using a really hard saddle solves this problem as is fitted correclty it only touches the sitting bones and because you cannot sink into the saddle padding it does not allow for touching anywhere else. So yes, most seasoned cyclist ride harder saddles not only because they are lighter, but because with time you realize they are more comfortable. Fit the one she has, angle the nose ever so slightly down, make sure she sits back on the saddle (move it forward if you need to) and feels that she is sitting on her sitting bones. Will probably be better experience for her. Good Luck!
