The Thudbuster seatpost has been THE CHOICE of suspension seatposts for tandem enthusiasts for decades. It comes with variable density elastomers to meet most stoker needs. I can't imagine your stoker wouldn't do well with this. If that fails, then perhaps the only solution for you would be a full suspension tandem. This would achieve the best impact attenuation she could achieve. Less unsuspended mass means the rear wheel can bounce in and out of potholes, up and over bumps and rocks with the most ease, allowing the rest of the bike and rider to be "insulated" from the impacts of the rough road surface. And this explains why the vast majority of mountain bikers today ride full suspension. It's so well designed and implemented that it's now the standard. Yes, it's overkill for "casual" tandem use, but if it's as bad for your stoker as your post indicates, then you may want to consider it.