In chapter 22, Mathilda (a.k.a., "Mattie") returns to Philadelphia and manages to reunite with Eliza. Eliza is surprised to see her. Mattie, however, is mostly concerned about the whereabouts of her mother. She thinks her mother has died of cholera and that Eliza is afraid to give her the bad news. When she asks Eliza about this, Eliza says,
No, no, she’s not dead. Don’t think that for a minute. Last I saw her, she was recovered from the fever and bent on following you to the farm.
Mattie's mother, though well, is missing, and it will be sometime before she and Mattie reunite. In the meantime, Mattie helps Eliza nurse others sick with cholera. Eliza is impressed with Mattie's dedication and her willingness to push herself to work harder, as that was not the way Mattie behaved before the cholera epidemic. Eliza sees that Mattie has matured because of having had to deal with adversity.
Mattie's maturity and growing decisiveness come into play again as she decides that she and Eliza will run the coffeeshop as co-owners. She makes the decision on her own, though it is supported by Mother Smith. By the time Mattie is reunited with her mother, she is a new person, no longer the child that she was.
It is important that Mattie have a period of time on her own, away from her mother, so she can mature without having a parent to depend on. At the same time, Eliza's information keeps Mattie from worry and grief.