Here the original text twice read “faithful in him”, where him refers to the Lord. For the 1837 edition, Joseph Smith edited the preposition in to to, which is what we expect in modern English. Nonetheless, Moroni 9:25 has retained the expression (“be faithful in Christ”). The same expression is also found in one of the witness statements:
three-witness statement and we know that if we are faithful in Christ we shall rid our garments of the blood of all men
Similarly, the King James Bible has an example using the preposition in for this expression:
There are no other examples in the Book of Mormon of “faithful to the Lord” except for these two edited examples in 1 Nephi 7:12–13. There are, however, examples of faithful taking the archaic preposition unto rather than the modern to:
All of these passages refer to one’s faithfulness to someone, yet the preposition is either in or unto but not to. Thus the critical text will restore the two uses of in in 1 Nephi 7:12–13.
Summary: Restore the phrase “faithful in him” both times in 1 Nephi 7:12–13 since the earliest textual sources read this way; similar (but unedited) examples of “faithful in X” (where X is a person) are found in both the Book of Mormon and the King James Bible.