you have to proceed very systematically and with caution. She must have been symptomatic. Has she seen a physical therapist. If yes, that would be your first line of inquiry.
If no, I would start with isometric exercises and have her learn to hold a neutral pelvis and then move on to very small movements like pelvic tilts. The next thing is to find what irritates the lower back. Moving into flexion and extension, she should tell you where her threshold is. Then you have the low back exercise stand-by exercises like bird dog, core movements on her back. When I have a client with a history of lower back problems, I stay away from any unsupported forward flexion.
It is really a constant dialogue between you and your client to find the best path.