You'll want to be sure she's not hanging onto bars too tight, pulling on them both, or putting too much weight on them. Be sure grip is relaxed and she can wiggle all her fingers, including the thumb. Weight should be on heels of palms, not requiring any squeezing of bars to support weight.
Also don't sit upright and bend over at stomach/back. Back and lower-back should be straight with any forward reach be done by bending and rotating of hip bones.
Doing back-strengthening exercises really helps back pains. When people do "core" workouts, they often do stomach crunches, which is fine if you're in feet-forward laid-back lounge-chair position. But in face-forward position, you want to strengthen back muscles, which is what holds your upper-body's weight up. I pulled/threw-out my lower-back seriously when i started racing BMX & motocross 35-yrs ago. Made it 1/2-season before i was bed-ridden for months.
Luckily i was still living at home! Doctors gave me pain-killers and PT/chiropractors gave me exercises to strenthen my back muscles. Roman chair back-lifts are most helpful, got to where i could do them with 25-lb weight behind my head! Next season, no sign of back pain or tiredness anywhere! When I got to university, I did serious bicycle road-racing and velodrome events.. with no back issues. It's combination of both form and strength.
Lots of stretching really helps too. Tight upper legs and hamstrings will pull on lower back muscles. I'll stretch, even for as little as 5-minutes, before riding down to corner store.
Interim quick-fix temporary solution would be to shave forward portion of seat-foam to move her forward more. And check out some clamp/bolt-on bar-options, particularly those that allow rotation of bars backwards. Another option is clamp-on bar blocks, allowing use of BMX/offroad bars with numerous rise and bend options (longer cables and brake line needed).
https://www.woodcraft-cfm.com/cgi-bi...&category=CORI
https://www.shopbrp.com/product-info...le-clamps.html
https://www.vitalmx.com/product/guid...ig/Triple,9027
https://www.revzilla.com/motorcycle/...-78-bar-clamps
I like this option best, easiest instsll with least misalignment possibilites. Strongest attachment and also anti-vibration clamping of bar for most comfort. I recommend adding reinforcement plate under factory triple-clamp.
https://www.ebay.com/itm/371509037865
