answer: Alex met Mary and they find out Cindy mentioned them to each other.
documents: #Person1#: Hey, where's Cindy?
  #Person2#: She told me that she's got tennis practice today.
  #Person1#: You're Mary, right?
  #Person2#: How did you know?
  #Person1#: Cindy told me about you in our gym class.
  #Person2#: I see. Do you live in this neighborhood, too?
  #Person1#: My house is next door to yours.
  #Person2#: Oh, Cindy did mention you before. You're Alexander Newman.
  #Person1#: Just call me Alex. Alex and Alexander are the same thing.