Never say never and always is always wrong. There is no one safe thing that will not be harmful to somebody. But we can't say "don't do anything" in order to be on the safe side, because doing nothing can often be the worst thing to do. So - what is a teacher to do?
I applaud your desire to find a way to help your student. As teachers, we do learn a lot when our students are in need. I know it has forced me to do a lot of research. I am not an expert on scoliosis and from what you have said, neither are you, so we must preface anything we tell a student with the standard "I am not a doctor, so please check out anything I suggest with a professional before trying it."
There was a long thread a while back discussing a scoliosis. You may want to read this
. The writer is a yoga teacher with scoliosis and her journey is quite interesting.
In the case of your student, you say that her condition is mild. If she is not suffering from chronic pain, then she may only need to find balance: one side of her spine will be strong and tight, the others side weak and lax. She may want to work through both Yin and Yang forms of yoga to mobilize the tight side and strengthen the weak side.
Yes, there are those who recommend "no twists" for scoliosis, or "no spinal work" - but since every body is different, these broad sweeping edicts are not helpful. You and your student will have to work together to find out what is right for her! Start easy: see how her body responds. If things are okay, go deeper, but take it in stages and ask her to keep paying attention to the results. Is it working? If it is - keep going, if it is not, back off and try a different approach.
Finally, I think you can do her a world of good if you encourage her to be the captain of her own soul. She should also be doing the research and not just relying on others to tell her what to do. You haven't said that she is or is not actively engaged in finding her own way to improve her condition, so I am not trying to judge her - just observing that, if she becomes the one in charge of her own health, she will benefit from this attitude throughout her life.
Hope this helps!