This is an example of face-saving behavior. She did this in order to avoid feeling embarrassed, or to lessen the embarrassing situation which may have happened if she showed that she heard them fighting. By pretending she didn't hear anything, she kept the appearance of a normal situation.
Mandela means that someone who decides to take someone else's freedom is prjudiced and narrowminded. He can not be free if he takes away someone else's freedom just as a person who does not have freedom.
A person who takes away someone else's freedom is not free.
