If you see a Christian brother or sister sinning then pray about their repentance
If you see a Christian brother or sister sinning in a way that does not lead to death, you should pray, and God will give that person life. But there is a sin that leads to death, and I am not saying you should pray for those who commit it. (1 John 5:16)
If you notice that a Christian brother or sister is in sin then pray for their repentance. Perhaps they indeed repent and come again to live for the Lord God. However not every time this is recommended because there are sins which lead to death. For instance, if you hear of a Christian committing suicide and dying, your prayers for his or her repentance won’t work since that person is sinned unto death. Do not despair if you hear of such sins such as suicide, selling soul’s to the Devil or even receiving the mark of the Beast. Rather give thanks to the Lord God that you are saved in Jesus Christ and that there are others in Christ who are still resisting sin!