So... is Arashi no Yoru ni: Himitsu no Tomodachi finished or not?
NO, it is not finished.
Why?
The anime finished after the manga:
Anime finished: September 26, 2012
Manga finished: December 22, 2005
... But MangaUpdates contains information about the anime adaptation and indicates that it is incomplete.
Watching Order (by release date)
Here you have all the anime relations we detected ordered by release date, so you can see the reason for this status in context and also use it as watching order.
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Synopsis
Arashi no Yoru ni: Himitsu no Tomodachi tells a story based on the book One Stormy Night... It revolves around an unlikely friendship that forms.
The anime explores the development of this secret bond between two individuals who, under normal circumstances, would be considered natural enemies.
Expect a tale about overcoming differences and the surprising connections that can arise in unexpected situations, set against the backdrop hinted at by the title: a stormy night.