It is wonderful that this new student has come to you. No doubt yoga will be a tremendous boon for her, helping with her polio disability and other, unanticipated areas. It may not be appropriate for her to attempt all the poses the class does at this stage, but over time, perhaps she will be able to do more and more.
Work with her to strengthen, stretch, and tone her legs. She should perform standing poses with support, so that balance is not a factor, and you will need to properly adjust and stabilize her alignment. When I refer to support, this includes a horse/beam, the wall, chairs, wall ropes, blocks, wedges—whatever is needed. Weights might also be appropriate.
I know this is rather general, but in such a specific case, it is improper for me to suggest more without seeing the student. I hope this gives you some ideas, and I would recommend that you consult with a senior teacher in your area.
Certified Advanced Iyengar instructor Dean Lerner is co-director of the Center for Well-being in Lemont, Pennsylvania and teaches workshop across the United States. He is a longtime student of B.K.S. Iyengar and served a four-year term as president of the Iyengar National Association of the United States. Known for his ability to teach yoga with clarity and precision, as well as warmth and humor, Dean has conducted teacher training classes at Feathered Pipe Ranch in Montana and other locations.