LDS scholars have a real answer here. Say so before your friend does.
3 Nephi 12-14 gives the Sermon on the Mount to the Nephites. It is near-verbatim King James English, matching Matthew 5-7.
Eight places where 3 Nephi follows late Greek manuscripts that scholars judge to be later additions. The clearest is the close of the Lord's Prayer at 3 Nephi 13:13 — "For thine is the kingdom, and the power, and the glory, forever." That line is missing from the earliest copies of Matthew 6:13.
The word openly at 3 Nephi 13:4, 6 and 18 is the same kind of case.
A record of Jesus speaking in the New World should not follow scribal additions made between the 300s and the 1500s.
John Welch of BYU answered at length. Several of Larson's eight are text-critically uncertain rather than settled.
And one runs the other way. 3 Nephi 12:22 drops "without a cause," agreeing with the earliest manuscripts against the King James.
Welch also points to deliberate changes for the Nephite setting, which mechanical copying would not produce.
Genuinely arguable. Welch's without a cause point is real, and you should hand it over before anyone asks.
The doxology and openly are still best explained by King James dependence.
"Why does the Nephite sermon include a line the earliest copies of Matthew don't have?"



Welch shows one variant runs the other way, matching the earliest manuscripts against the King James.
John W. Welch of BYU responded at length. Several of Larson's eight variants are uncertain rather than settled. One runs the other way. 3 Nephi 12:22 leaves out 'without a cause', agreeing with the earliest manuscripts against the King James of Matthew 5:22.
Welch also argues that the King James wording elsewhere reflects the register of the translation. It does not reflect the Nephite record. And 3 Nephi adapts the sermon to its setting on purpose. Mechanical copying would not do that.
Arguable both ways: Welch's reverse case is real, but the added prayer line is not.
Welch's reply engages the variants seriously. The missing 'without a cause' at 3 Nephi 12:22 is a real point for them. Hand it over before anyone asks.
The prayer ending at 3 Nephi 13:13 and the word 'openly' are still best explained by King James dependence. Scholars do argue over single cases. The King James register across the whole sermon is not in doubt.