Good question Rene!
Matthew 5:14-16 says....
Ye are the light of the world. A city that is set on an hill cannot be hid.
Neither do men light a candle, and put it under a bushel, but on a candlestick; and it giveth light unto all that are in the house.
Let your light so shine before men, that they may see your good works, and glorify your Father which is in heaven.
There are definite moral grounds for speaking to friends and to others we know or meet about their actions and therefore there is no harm in telling folks if you think they are doing wrong. Even non-Christians do that.
In regards to sin, there is the need to put it in perspective of God's grace. Jesus didn't go around telling folks they were sinners, but rather, offered them escape and deliverance from the power of sin in their lives. He offered hope and life. Those who had ears to hear, meaning those who would listen to His words, heard what He said, and understood that He offered them more than the world could offer.
To go and tell someone they are sinning, we need to hear also, to hear the prompting of the Lord to do so.
For me, I had no idea what sin was. What I saw was do-goody Christians trying to make me and others feel guilty for not being perfect like they thought they were..... I had no idea what Jesus was about. That was my ignorant interpretation of things.
It was only AFTER I became a Christian that I understood the full ramification of sin.
Before then, I was a law unto myself and I had harsh words for those who tried to tell me their opinion on things... especially any judgemental sort of stuff.
Friends though, will take time to help each other and if that means sitting down and explaining with love about sin and righteousness found only in Christ, then it is a mark of true friendship. A friend will listen, at least, to you.
But just telling someone they are sinning, well...... not sure how that would go down, meaning I am not sure you will be listened to, but could actually cut yourself off from them, and then not be able to share anything better, like Jesus Christ the Lord and His gift of eternal life.
Bless you ....><>
Br. Bear
John 6:44 No man can come to me, except the Father which hath sent me draw him: and I will raise him up at the last day.