@qornanali The Japanese aren't quick to "warm up" to other people, so they reserve the first name calling for a closer level of intimacy (BFF/boyfriend/girlfriend/family).
@egie It is, and in some circumstances (like here) even more. As childhood friends, as kids they didn't care. But as they rounded Jr High, either social conventions or classmates teasing "forced" them to go to the standard surname. So now they'd be going BACK to first name, which would be a pretty explicit "fuck you all, we're THAT close".