David's lament over Jonathan in 2 Samuel 1 is one of the rawest expressions of grief over the loss of a friend in all of Scripture. "I am distressed for thee, my brother Jonathan: very pleasant hast thou been unto me: thy love to me was wonderful, passing the love of women." David had enemies in abundance and had learned to carry military loss. The loss of Jonathan broke him differently. Some griefs are in a category of their own.
Psalm 55 is the other side of this — David's anguish not over a friend's death but over a friend's betrayal. "For it was not an enemy that reproached me; then I could have borne it... But it was thou, a man mine equal, my guide, and mine acquaintance. We took sweet counsel together, and walked unto the house of God in company." The specific wound of betrayal by someone who knew you, who walked beside you, is something the Psalms refuse to domesticate.
The New Testament gives no formula for this grief either. What it offers is presence — the Holy Spirit as *Paraclete*, one called alongside, who comes near to the places where human companionship has failed or ended. You are not alone in the loss, even when the loss is precisely of not being alone anymore.
Commentary is from a charismatic Protestant perspective, drawing on KJV text and public-domain sources including Spurgeon, Andrew Murray, and Matthew Henry.