Read Nicki Doane’s Response:
First and foremost, I would insist that she consult her oncologist before going to any yoga classes at all. After she has received the OK from her doctor, then you and she can figure out what will work best for her. This may involve private lessons for a while, or possibly figuring out a way of modifying certain poses so that she can still come to class.
The most interesting part of your query was your statement that her memories of the class making her feel good are what propelled her to return. It may be that the class, especially if it is a strong practice or vinyasa in style, will not make her feel better. If she gets nauseous or light-headed at any point, I would ask her to stop.
I teach a weekly restorative class, and that is the kind of class I would encourage her to attend. Start slowly and add a little bit at a time. If something doesn’t feel right to her, it probably isn’t.