Read Timothy McCall's response:
Halasana (Plow Pose), perhaps more than any other asana, causes a flattening or even a reversal of the normal inward curve of the lumbar spine. When the low back curve is reversed, the front sides of the lumbar vertebrae come closer together and the back edges separate. This compresses the front side of the spinal disc between the vertebrae, which may cause the disc to bulge backward. A bulging disc can push on a nerve exiting the spinal cord nearby, which is the most common cause of sciatica. This is why Plow Pose is generally contraindicated in that condition.
My guess is that your student is feeling pain either from compressing the front edge of the spine and nearby structures or from compression of a spinal nerve by a bulging disc. Neither one is good. It may simply be that your student lacks the flexibility to do full Plow Pose. This can be remedied by having the student bring her feet down onto the seat of a chair or a bolster instead of going all the way to the floor.
Even with the feet elevated, some students may still slump the front torso, so remind her to lift the front thighs toward the ceiling to help elongate the front side of the body. Bending the knees can also take the pressure off the spine. If these measures prove insufficient, it's better for her to skip the pose entirely.
If a student develops back pain due to reversing the lumbar curve, gentle backbends such as Bhujangasana (Cobra Pose) or Salambhasana (Locust Pose) may be useful in reestablishing the normal curve.