Triumph seat length problem

As shown in image, when my friend and I sit on bike, most of the time she will be sitting on the curve part of the seat and it causes her tail bone to pain. I've a small torso so I would be occupying half portion of my seat so we have lot gap between the curve section and my back. She don't intentionally sit on curve part rather when I apply brakes she slides down. She isn't comfortable holding grab rail, so can't hold on to that. Because of this she has to adjust herself every other minute. This is causing both a bad riding experience. Can anyone suggest what is the best solution to this? I have watched couple of videos where they add more form to curve section reducing length of rider seat and thus increasing pillion seat length. Is it worth to make such changes?