Adam Clarke on Proverbs 27:5

Open rebuke is better than secret love - Plutarch gives an account of a man who, aiming a blow at his enemy's life, cut open an imposthume, which by a salutary discharge saved his life, that was sinking under a disease for which a remedy could not be found. Partial friendship covers faults; envy, malice, and revenge, will exhibit, heighten, and even multiply them. The former conceals us from ourselves; the latter shows us the worst part of our character. Thus we are taught the necessity of amendment and correction. In this sense open rebuke is better than secret love. Yet it is a rough medicine, and none can desire it. But the genuine open-hearted friend may be intended, who tells you your faults freely but conceals them from all others; hence the sixth verse: "Faithful are the wounds of a friend."