# whoever turns a sinner from the error of his way ... will cover over a great number of sins

James means that God will use the actions of this person to persuade the sinner to repent and be saved.

# will save him from death and will cover over a great number of sins

"will save him from spiritual death, and God will forgive the sinner for all of his sins"

# will cover over a great number of sins

Possible meaning: the disobedient brother, when he returns to the Lord, will have his sins forgiven. Sins are spoken of as if they were objects that God could cover so that he would not see them.