My mom was in a car accident and now she has back pain and numbness and weakness in one arm. What questions should I ask the doctor/be concerned abt?

BACK PAIN AND NUMBNE. If she has numness and weakness in the arm related to the auto accident then the pain in back is high up in neck area. If she has pain in lower back as welyou need to be concerned for pinch nerve in neck area (cervical spine) causing the numbness and pain in arm and extent of back injuries and she needs MRI exam to confirm the clinical diagnosis and may need meds and/or physical therapy.
MVA pain. Your mother needs a good, basic neuro exam and a CT of the head and neck. Most likely she has some cervical nerve compression in her neck on the same side as the weakness. Fracture/dislocation is possible. It is also possible that she stretched/damaged a major nerve bundle in her upper arm. If she has low back pain that is a separate pathologic issue but also might need a CT.