Florence mentions her grandmother's saying, "Be Not Solitary. Be Not Idle," to suggest the importance of both meaningful relationships and purposeful activity in achieving a fulfilling and happy life. This saying encapsulates the idea that to lead a rewarding life, one should engage with others while also being involved in productive work. Therefore, the most fitting interpretation of her mention is:
Happiness is achieved through useful activity and meaningful relationships.
This perspective highlights how social connections and a sense of purpose contribute to overall well-being.