Is what a mother offers her daughter considered charity or a duty?

A divorced mother falls into two categories:

1. If she is poor and unable to support herself: Her wealthy children must spend on her, and it is not permissible to give her zakat.

2. If she is wealthy and able to support herself: Her children are not obligated to spend on her as a fard (religious duty), but giving charity to her is permissible, and she has a greater right to it.