No. As far as the repentance gate is open for every one as we read in Quran, we can never claim that a person who committed a sin is a permanent sinner. As a witness of his committing the sin in front of you, you need to verify whether he is still committing sins or not before praying behind him. You are not allowed to tell others not to pray behind him nor inform them about the sin which you witnessed unless he did it openly with out caring to public seeing him committing the sin.
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