"A mistress never is nor can be a friend. While you agree, you are lovers; and when it is over, anything but friends"

This quote suggests that a mistress, by definition, can not be a real buddy due to the fact that the relationship is based upon an agreement to be enthusiasts instead of a deep, meaningful connection. As soon as the romantic relationship ends, it is not likely that the 2 individuals can preserve a relationship, as there was never a structure for one in the first place.