L.B.
Your question is very interesting. My daughter has dyslexia, but does not read or write in a mirror image. I have heard about kids doing that. The gold standard teaching for dylexia is Orton Guillingham Method. The method was developed by the doctor who taught Helen Keller to read and write. Orton Guillingham method has been refined since Helen Keller times. The technique has proven to be the most effective intervention for dyslexia. Many knock offs are used in the school system, such as the Wilson Method. In my opinion, not as good. If you would like info on some resources send me an e-mail.
How did you realize that your child was reading and writing in mirror image format?