Everybody knows each other.
(2b) They know each other.
I did. Maybe you didn't since you didn't understand the reference I made to it. Everybody is not referring to one person but a group of people. Hiltz is one person. The article is loaded with errors. Everybody knows one another. They know one another. It is not singular. Hiltz is one person. She is the correct pronoun. Every 5th grade English teacher knows this.