I've had lower back, hip, and knee issues. The fix for the hip pain isn't so much a straighter back, it's opening up the hip angle. this can be done in several ways...a combination of fore-aft, seat angle/and seat post height. I've definitely set off sciatica attacks by riding with my hip angle too narrow. It's the old hip bone connected to the thigh bone song. You can't just change one thing without it impacting everything else. A posture with more lean will open up the spaces between the discs, a straighter back will decrease the space and potentially be more jarring. If you change the reach to put the back up higher, that will change other measurements. I highly recommend that you get a good bike fitter to first make sure she is sitting in a way that optimizes her hip angle and balances her core as well as possible. THEN, and only then, should you determine what a comfortable reach would be and what adjustments can or should be made. It is a very complicated process, particularly when on has some physical issues already, and a good fit will be worth every penny. Make sure the fitter gives you a re-check about a month out to make final adjustments. FWIW, I had my seat off about 1.5cm and created a severe problem with one knee over such a tiny difference; it actually took over a year to resolve the damage. Now I am extremely careful to make sure I follow my measurements and check my positions regularly.