It sounds as if the mother regrets her decision and now would like her daughter to have the education she may feel she did not achieve. It seems so simple at face value, just let her go back, but I Imagine that could set a precedent to others who want to “go back and redo” high school for a thousand of different reasons.
Therefore, I believe there could be other options. Perhaps they could find a volunteer tutor to pick up where she left off and help her. I bet someone would be willing to help if they knew of the circumstance. The lawsuit seems extreme and unwarranted.
I am not sure that it would set such a precedent. Part of the reason for her wanting to go back is because of the traumatic brain injury. If they could set things up so that only people who have experienced such things could go back then I would see nothing wrong with it.