I have had a large number of students over the years with sciatica. There are a number of yoga positions that are helpful, which may be one reason so many people with sciatica seem to show up in class.
The first thing to remember is that a personal trainer or a yoga teacher (except those with yoga therapy backgrounds and then in defined situations) does not treat. When I am approached by someone who mentions a history of this condition I always inquire about current and past medical care. I generally encourage them to speak with a doctor or a physical therapist to get their input on what they should do or avoid. I also suggest they ask for a handout…. some physical therapists may have those.
In a way this is a good opportunity for you to make some connections with your local medical community. You might have a letter to send with her to her care giver letting them know you will be working with them and asking for any feedback they would wish to give. Or you could see if there are local physical therapy offices that might be willing to give you some information. The main thing is that if they are currently undergoing treatment it is important to get clearance to work with them, and to have a defined list of things they may and may not do from the medical professionals.
Once I have made sure the person doing the class is in the post treatment stage I offer them some individulaized guidance in class of things people have found helpful in my experience, always with the caveat that if my suggestions either go against what the doctor recommended in their case, or does not feel good for them they should not do it.
I would not recommend you do yoga with them yourself. Even Judith Lassater (an extremely experienced yoga teacher and physical therapist who is an expert in therapeutic yoga) recommends a yoga teacher be quite experienced before working with a student with sciatica. However, this is a link from the Yoga Journal (not quite a scientific source, but good for general info) on the subject:
And if you have a Viniyoga studio in your area you might refer them there, as they are really good for this kind of practice.