There is a special obligement upon friends to be helpful to one another… The laws of friendship require a discovery of that which endangers one another. You would count him unworthy the name of a friend, who knowing a thief or an incendiary to lurk in your family, with a design to kill, or rob, or burn your house, would conceal it from you, and not acquaint you with it on his own accord. There is no such thief, murderer, incendiary, as sin: it more endangers us, and those concernments that are more precious than goods, or house, or life; and that most endangers us, by which the Lord’s anger is already kindled against us. Silence or concealment in this case is treachery. He is the most faithful friend, and worthy of most esteem and affection, that deals most plainly with us, in reference to the discovery of our sin. He that is reserved in this case is but a false friend, a mere pretender to love, whereas, indeed, he hates his brother in his heart (see Lev 19:17).
The Works of David Clarkson, 2:216.