Tattling or reporting a wrong is a serious issue. We are all sinners and are thankful no one is reporting all of our errors to others. If a sin is very serious, however, then you need to weigh the matter in your mind and pray about it. Will the sinful behavior seriously hurt others? Are stealing, adultery, fornication, compulsive lying, or leadership sins involved? In matters like these, it may be wise to ask an older, more mature Christian for his or her perspective on the issue. When discussing the problem, don’t use names. But present the situation theoretically and ask for an opinion as to whether or not it needs to be acted upon.
If you do feel something needs to be done, then Matthew 18:15-17 is the procedure to follow. First, go directly to the sinner and try to reason on the issue. If he refuses to listen, the Lord tells us to bring one or two more to discuss the matter. Finally, if all this fails, it should be brought before the church.
A true friend will certainly talk directly to warn those he loves. "Faithful are the wounds of a friend…"Proverbs 27:6 However, it would be ill-advised to intrude into another's business unless you already have a close, trusting relationship. It could sound like busy-bodying to the hearer.
Consider Jesus's example. He could have been warning and pointing out sinful behaviors all day and night. Instead, He spent His time comforting, encouraging, blessing, preaching forgiveness, and healing. Wise behavior like Jesus's will grow loving relationships with others. Then you will be in a position to occasionally advise/rebuke a friend.