Read 1 John 5:16 Using Other Translations
If any man see his brother sin a sin which is not unto death, he shall ask, and he shall give him life for them that sin not unto death. There is a sin unto death: I do not say that he shall pray for it.
If anyone sees his brother committing a sin not leading to death, he shall ask, and God will give him life--to those who commit sins that do not lead to death. There is sin that leads to death; I do not say that one should pray for that.
If you see a fellow believer 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.