I have been a passenger on many occasions. By far the most comfy ride was being on the pillion of the Valkyrie. (Unfaired Goldwing basically). It had enough room on it to not feel squished. I could lean back against the back rest and just enjoy the ride.
The second most comfy ride was on the Royal Star Venture. The only problem for me on this one is that on a long ride, my knees start to hurt. If a person has shorter legs it would not be an issue.
The 1000cc Virago was too short for comfort on a long ride. It was ok for a while because of the back rest, but there is no room to move the torso slightly for a change of position on the back.
The V-Star 1100 was ok, but the pillion seat is like a plank. The seat would have to be re-done for any kind of comfort.
One of the big Harley's I was on as a passenger was comfortable enough ergonomically, but the vibrations were awful. Too much vibrations on a back back will likely increase the discomfort.
The pillion seat should be large/wide enough to support the whole butt. If the seat is too narrow the body will be trying harder to keep balanced causing more strain on the back.
In the meantime, a back support (like a weight lifter belt idea) can help a great deal. I speak from experience. Since my crash three years ago, I have had ongoing problems. I could not ride for very long without my back/hip/shoulder hurting. I started wearng a back brace for some rides and it made a tremendous difference.
Similar to this....she could choose the one that is most suitable for her. There are several styles. The one in the picture is about $35.00
Aside from the backrest, back brace, good pillion seat.....smooth shifting, acceleration and braking will all help to make a difference. If you brake suddenly or shift rough or accelerate abruptly, it forces the back to be jarred....(this is where the back brace will help).
Avoid ridged and bumpy roads when possible and check your suspension. A softer ride may also help.