Question
I have a friend (Jane) with low back pain and have no problem with training. Lately her LBP getting worse due to her new job in retail and she can't do her normal bench press. From her standing posture, she have long thoracic kyphosis and anterior hip shear (flat back posture type).
Q1: Why sit-ups or crunches would not help her low back pain, but may even make it worse?

Q2: Her hamstrings also seem to be getting very tight with jogging in my sessions....why might this be?

Q3: Which area could be loosened on her to help with the pain?

Q4: How would I retrain her posture? What cues might work best
for her on the job?

Answer
She should have her low back examined to see what is happening with the vertebrae and discs, she might have a short leg causing twisting of the spine and an unlevel pelvis. Have her stretch the spine on a large therapy ball. She can also do crunches and lower back extensions on the ball. Stretch out the quads, hip flexors(illiopsoas), gluts and hamstrings. Have her see a Chiropractor(First Choice) or Physiotherapist if the pain persists.