*Notice - Brethren, if anyone “among" you wanders from the truth..turns a "sinner" from the error of his way.. Some would argue that James says this one who turned from the truth was a "sinner," and was "among" but "not of" the Brethren, then he wasn’t previously saved. That fits 1 John 2:19 - They went out "from" us, but they were "not of" us..
IF this person was a genuine believer, yet how do we know for sure this is the second death in the lake of fire? In Matthew 26:38, Jesus said: "My soul [psuche] is deeply grieved, to the point of death." Jesus was not saying that His soul was deeply grieved to the point of spiritual death, Rather, Jesus was talking about physical death, his human life.
In Revelation 16:3, "The second angel poured out his bowl into the sea, and it became blood like that of a dead man; and every living soul [psuche] in the sea died". In 1 Peter 3:20 "... God kept waiting in the days of Noah, during the construction of the ark, in which a few, that is, eight persons, [psuche-souls] were brought safely (saved from drowning, physical death) through the the water" by the ark (Hebrews 11:7).
"Soul" is rendered from the Greek word "psuche" and is also translated as "life". The word "psuche" is never translated as "spirit." Now Jesus covered our sins in one way (Romans 4:7) by bringing forgiveness for all believers, yet sins can also be covered in a different way.
In Proverbs 10:12, we read: Hatred stirs up strife, But love covers all sins. In 1 Peter 4:8, we read: And above all things have fervent love for one another, for "love will cover a multitude of sins.” Where there is strife, there is hatred and unless love prevails, the strife will get worse. Love covers offenses and sins when a believer turns back from error.
So is this wanderer a professing Christian, whose faith is not genuine, or a sinning Christian, who needs to be restored? For the former, the death spoken of in vs. 20 is the "second death" (Revelation 21:8); for the latter, it is physical death (1 Corinthians 11:29-32; 1 John 5:16).