Read Timothy McCall's response:
Scoliosis is a complicated condition for which one-size-fits-all suggestions of "helpful" poses don't always work, as you've seen. Although we tend to notice the sideways curvature of the spine in scoliosis, there is also always a rotational component as well. If the curve involves the thoracic spine, as is common, the ribs on the concave side will typically be contracted and those on the convex side expanded. Regardless of where the primary curve is located, it's also common for there to be a second compensatory curve elsewhere in the spine, and for the pelvis to be misaligned. The precise nature of what's likely to be helpful will depend on how severe the scoliosis is and where the curves are located. This is something that requires a trained eye to accurately diagnose.
If you feel qualified, I would suggest planning a private session with the student so that you can spend more time on an evaluation than would be possible in a class setting. Then, based on what you see, you can try out a few practices and see how Ann responds. Poses such as Adho Mukha Svanasana (Downward-Facing Dog), or Half Dog with the hands on a chair or wall, can be useful to create more space between the vertebrae. Asymmetrical poses, such as standing and seated twists and side stretches, can be particularly useful—but you'll have to pay special attention to make sure they're right for her.
You will want to avoid poses that stretch the expanded areas any further. Sometimes a muscle may feel tight and painful because it is overstretched. Stretching it even more could make the pain worse. It's possible to learn to do both symmetric and asymmetric poses in ways that compensate for the compression and rotation without increasing the unwanted expansion, but this takes precise instruction on your part and good bodily awareness on the student's.
If any of this seems daunting or beyond your training, I would suggest you refer Ann to a more experienced teacher. By seeing what this teacher recommends, you may get ideas of how to work with Ann in class. Or perhaps you'll realize that a class setting is not the best place for her, and that her focus should be on a daily home practice of those carefully chosen poses that prove most useful. Good luck!