Paul makes a great distinction in these verses. If a brother—a fellow Christ-follower—doesn’t follow the Bible’s commandments, then we need to not associate with him. There’s a risk we take when we are intimate friends with a backslidden or disobedient Christian, and Paul exhorts us to avoid that intimacy.
However, we’re not to shun him or “regard him as an enemy.” We certainly shouldn’t gossip or point fingers. Again, we’re not to judge him (Matthew 7:1-3). Instead, we should “admonish him as a brother.”
Jesus Himself gave instructions with how to deal with a brother (or sister) who has strayed:
Moreover if your brother sins against you, go and tell him his fault between you and him alone. If he hears you, you have gained your brother. But if he will not hear, take with you one or two more, that “by the mouth of two or three witnesses every word may be established.” And if he refuses to hear them, tell it to the church. But if he refuses even to hear the church, let him be to you like a heathen and a tax collector. (Matthew 18:15-17, NKJV)We need to do everything we can to bring a prodigal brother (or sister) back to the fold while making certain we ourselves aren’t pulled away.