Jade Maddex and the Prisoner of Azkaban

An escaped prisoner from Azkaban, Sirius Black, who's believed to be a follower of the Dark Lord, is after Harry. Jade writes to her mother about her concerns but her mother tells her not to worry about Black, that he would never harm Harry. How does Jade's mother know this man? How was she supposed to focus on school when there was a murderer after her friend?