So she was told by her doctor to sit up straighter. I would have to respond that our family doctor is a long distance cyclist. I know of no other MD who is competent to prescribe bike position. Just sayin'. Seattle is lucky enough to have a facility which combines medical and cycling knowledge, Corpore Sano, but I know of no other.
That said, the idea is sound: we function best with straight spines, however that statement does not address spine angle. It is however well known in the cycling world that sitting up straight imposes high compression loads on a straight spine, and that's not a good idea. There's a good reason that the standard road position involves leaning forward at about a 45° or greater angle. The reason is that in that position the spine is less subject to compression loads. All the road cyclists I ride with know well that the best thing for their backs is to spend a lot of time on the bike, in the road position. The reason for the forward lean is not so much about aerodynamics as so many people believe - it's about improving comfort and the ability to operate the pedals.
The best road position is achieved by rolling the pelvis forward, off the ichial tuberosities and onto the pubic arch. The back should be approximately straight all the way from the top of the shorts to the back of the skull. This also gets rid of neck pain. This straight back should be at about a 45° angle to the horizontal See:
https://www.bikeforums.net/road-cycl...discovery.html
Reach to the bars is correct when, with hands on the hoods, the elbows bent, and the back straight, the upper arm makes a 90° angle with the straight torso. This way the arm functions as a strut which requires less shoulder effort to hold the position.
It may be that she'll find rotating her pelvis forward in this manner to be uncomfortable, in which case she needs a saddle with more of a cutout, something like a Selle Italia Diva.
This is what I'm talking about.:
