I think anyone who teaches yoga will see people with various spinal conditions. There is a lot in yoga that can be helpful, and the way that we use dharana and pratayahara can be a huge aid in avoiding injury by keeping focus and moving with intention.
The problem with giving an answer in this forum is that I have neither seen this person, nor have I read her medical history, nor talked with her. I do not feel that it would be professionally appropriate to make specific recommendations. I would like to offer you a few general thoughts I have that I think might be of some help.
Before you stretch or strengthen her it would be of enormous help if you have her take a release form to her medical advisor. This helps you legally, but more than that, it gives you a framework of what medically is the baseline NOT to do.
When I work with someone who has rather serious back problems I often send them to a senior Viniyoga practitioner I know. This is someone who is trained in yoga therapeutics. I guess my point is, I am an ERYT 500, but even with that, I am not an expert on yoga therapeutics, and there are times I like to refer someone on.
A lot of Viniyoga people do workshops on back care, and you might wish to suggest this to her, or you might even yourself find it helpful to take a class like this. You might find that such training would enhance your work with her, and with others who might come to you with similar situations. Or you could look for some books by Gary Kraftsow. I took a week workshop with him and if her were closer to me I would go for a long training. He worked with Tirmuli Krishnamacharya himself and is really one of the top people for this sort of healing yoga anyplace.
If you are looking for a really good book I have 2 suggestions. The firs is basically anything by Judith Lasater. She is a physical therapist and very senior yoga teacher. Her yogabody is on my shelf and I find it very helpful.
The second is not a light read for the beach, but very very good to read short bits. It is called ‘A Physiological Handbook for Teachers of Yogasana’. If you want detailed science and anatomy as it relates to yoga practice this is (in my mind) the gold standard.
btw, in what school of yoga did you train? I looked at your profile but it didn’t say. (it is easier to give an answer if I know someone’s background, so I don’t make assumptions, or tell you things you know better than I do)