Translation of "Confidant" in Japanese

He is my boss's friend and confidant.

彼は私の主人の腹心の友。

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

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。