You should definitely not stop your granddaughter from writing in this way, or even make her correct it. If she asks if it is correct, you should point out the errors, but don't make a big deal of it. The more that children write, the easier it becomes for them. You don't want to limit your granddaughter's creativity by constantly pointing out what is wrong. When she learned to walk, you wouldn't have dreamed of telling her to stop because she couldn't do it properly, or of trying to correct her and give her lessons. Similarly, with writing, children should be free to practice and make multiple mistakes without corrections or limitations. By first grade, teachers will start asking children to correct their reversals, and by the end of second grade almost all children have stopped doing it completely.
Five-Year-Old Writes Backwards
Pediatrics Expert Advice from Shari Nethersole, M.D.