腹心

彼(かれ)は私(わたし)の主人(しゅじん)の腹心の友。

He is my boss's friend and confidant.

彼(かれ)の腹心の部下が彼(かれ)を裏切って、彼(かれ)の出世の邪魔をした。

His right-hand man gave him away and got in his way to the top.

汚い手で会社を乗っ取ったものの、腹心だと思っていた部下に裏切られ、因果応報だね。

When the trusted confidant of the person who conducted a hostile takeover of the company betrayed him, it was a sort of retributive justice.