The author goes back to the singular again—the enemy of Psalm 55:13–14. This enemy is his companion, his familiar friend who stretched out his hand against his friends. The author is not alone in this. The previous confidant has broken his covenant; his bond of friendship has been violently shattered. There must have been something like an oath or a promise of loyalty. But he trampled it underfoot. Yes, the relationship has turned upside down—the friend turned into the enemy (a desecrated friendship).
The covenant of the Lord with his people has meaning for the mutual relationships of people amongst each other, see Malachi 2:10: Have we not all one Father? Has not one God created us? Why then are we faithless to one another, profaning the covenant of our fathers?
20 My companion stretched out his hand against his friends; he violated his covenant.