Mine complains of that too. With her I kind of thought that she was nervous (she has very little time on a bike) and holding on to tight. One thing that we are going to try is a backrest. I know of two options for a backrest. You can get a corbin seat and add a backrest. You could also get a Givi topcase and add a backrest. I just got the topcase in the link below but we haven't had a chance to try it out with her yet. We have two little kids so we rarely go someplace together without them.
Is the back seat slanted downward (I've never sat on it). If so, I had that same problem with a front seat that slanted downward toward the tank. You have to tense your muscle and sit with a funny posture when seats are slanted downward (that is, noticably downward).
I would look at the padding in the pillion section of the seat. It may be angle just right to instill enough stress at just the wrong way on her spine. A quick item to try that is fairly inexpensive is a seat pad. The have inexpensive ATV style pads at Wal-Mart that is popular with KLR and Adv riders. It's a full pad cover, but may help. If you go to a H-D dealer. They have smaller pillion gel pads that are small enough just for your position or you can get the large one the will over lap the side of the seat.
In summary. Quickest and cheapest, try a pad. The I would go with a Givi with gel back pad. The storage is always good. IMHO, it looks good on the bike. It would offer her a bit better feeling of security. The full monty would be get the seat redone and add the Givi.
Here's the Givi V46 with gel pad back rest and luggage rack. It held up very well and I woudl recommend the you get it from Twisted Throttle with the quick attachment plate.
It's better to have loved and lost than live with the psycho for life!
Walmart has a seat pad that is molded gel in a honeycomb pattern. It has a suede type and mesh cover. It is about the size of a chair cushion, and the gel inside can be cut down to custom size if needed. Best of all it is only $18. I used this on my previous bikes and it is the best money I have spent. It is super comfy. You just put it on top of your seat like those sheepskin and beaded pads that they sell for over $100. I would use this on my 919 if the seat wasn't so tall already. Its about an inch thick so if your tall enough to use it then it can make a big difference in comfort. I'm guessing it would work for a passenger too. They keep them in the automotive section.