Is it permissible to throw dust on the face of the deceased?

It is not permissible to throw dust on the face of the deceased. What is prescribed is to gently toss soil onto the grave three times after burial. If she did that without believing it was prescribed, or unintentionally, there is no sin upon her. However, if she did it out of idleness or disrespect towards her mother, then she has committed a great error, and she must repent, seek forgiveness, and supplicate for her mother.