Even as non professional, I am slipping in the spot where I interact and teach youngsters, I do not want to make grave mistakes that could be easily avoided by a little more background knowledge.
What are the grave mistakes? It's a riding lesson. If you are talking about basic safety and the student is not capable, then again, I would tell the parent to seek out private lessons. Whether or not she has a disability (and/or knowing what it is) really isn't going to help. If they tell you she is autistic -- how does that help? Can you offer her private lessons now?