The Busa, even in it's softest setting, might be way to harsh for her. On pillion, she's sitting almost directly on her tailbone, which guarantee will be painful if she's got any problems.
My wife is the same way. For right now, we really don't ride together much. I did borrow an Electra Glide from a friend once, and she LOVED that! (even if I was not really feeling it).
Any chance you can borrow another bike? A big cruiser or touring bike?