Who is Mrs. Lizzie Tristam from The American and what is their importance?

Lizzie is Tom's wife, and over the course of Newman's stay, they become close friends. After Newman returns to America, Lizzie continues to write to Newman and
send him news of Claire. At the close of the novel, it is Mrs. Tristam who suggests to Newman that if he had really pushed the issue of the Bellegardes' secret, then they might have allowed him to marry Claire.