What does Lily's mother tell her about the time she met Atlas when Lily and Atlas were teens?

Lily's mother tells her that she met Atlas once. She came home from work early and Lily and Atlas were asleep on the couch. She was about to yell at them when Atlas woke and he looked scared. Not of her, but that he might lose Lily. He left quickly, and she followed him outside. She planned to threaten him, but then he gave her a hug like he carried genuine sorrow for her as if he was encouraging or comforting her. Then he walked away.