This is Chapter 10 in Volume 3 of A Place Further Than the Universe
The expedition team moves into the research base in Antarctica.
Yuzuki gets an e-mail from her mother. She’s gotten a role in a morning TV drama. She’s concerned that she won’t be able to hang out with the other girls if she accepts. Kimari blurts out that they are all best friends anyhow, which gets Yuzuki upset. With no experience having friends, she hadn’t noticed when that happened. She asks them to sign a friendship contract, and needs it all explained to her again, but continues to struggle with understanding what friendship is.
The evening after the Christmas party, Kimari shows Yuzuki her texts to Megumi, and explains how their friendship works. Then they present Yuzuki with a birthday cake, since Yuzuki’s birthday passed while they were seasick. Yuzuki once again is so happy that she cries.
This chapter is based on Episode 10 of the anime.