My family is glued to the TV every week watching American Idol. It’s great entertainment watching person after person think they can sing only to sound like an moaning, injured animal. You’re left asking, "What kind of friends do these people have?" "Why didn’t anyone tell them the truth?"
Is God asking you to reveal someones blind spots to them. Let us take a cue from Jethro in Exodus 18. Moses’ father-in-law celebrated what God was doing in and through Moses. He worshiped with him and he affirmed their covenant relationship. (vs9-12) This is what also allowed him to tell Moses the truth about what was not good in his life.
Those are the elements that will make a safe "truth teller" to our friends I think that if we are not in a relationship with someone in these ways then we should not take the liberty to point out what is wrong in their life. When we celebrate, worship and covenant with someone we have established a relationship where God and his love keeps us together in grace and truth.