Watashi no Otto wa: Ano Ko no Koibito

Sasano Kaori and her husband are very close but Kaori starts to notice that her husband has "changed". One day, while talking to her best friend Yuki, Kaori learns that Takuya has a secret account with many posts that hint an affair. Kaori later finds out that the partner is Mishima Mutsumi and visits Mutsumi's husband, Kyosuke, and asks for his cooperation in separating Takuya and Mutsumi. Kyosuke agrees with the condition that Kaori sleeps with him. While following and monitoring Takuya and Mutsumi, Kaori begins an affair with Kyosuke, and later falls in love with him.
(Source: DramaWiki)
~~ Adapted from the manga series "Watashi no Otto wa: Ano Ko no Koibito" (わたしの夫は―あの娘の恋人―) written by Aizawa Atsuko (あいざわあつこ) and illustrated by Gin Tsukishiro (ツキシロギン).