Whether she's talking about motor vibration, or road impact or both, I'd bet the reason she feels more of anything than you do is the passenger's weight is supported by prolly no more than 1/3 as many square inches as the rider's weight, years ago when my wife would ride with me, that was her compliant.
Myself, I don't care for the larger passenger seat section of the Sundowner seat, since she's not riding with me these days, but you might not have a better choice if what she needs is more area of seat cushioning.
A temporary padding test made out a couple of layers of a cheap rubber-backed bathroom floor mat draped over the seat would stay in place really well, and until it got really well squashed, might give her an idea if a bigger seat area is necessary, or if better padding alone might do it.
Maybe a nice fuzzy pink one?