Musar zu Divrej Hajamim II 20:34

The fourth category of flattery is he who becomes a companion to the wicked. Even though he does not flatter him and does not praise him, but since he is near to him and in his company, he will be punished. Not only does he not rebuke him, but on the contrary, he brings him near to him in companionship, and puts him at a distance from his rebuke, and there is a sin in this. He adds to his sin by bringing the wicked person so close to him, as it is said, "Because thou hast joined thyself with Ahaziah, the Lord hath made a breach in thy works" (II Chron. 20:37).