In his editing for the 1837 edition, Joseph Smith first tried to change the generic plural pronoun they to the singular he so that the pronoun would agree in number with the preceding such an one. However, the following clause “for they are the children of the kingdom of the devil” could not be changed to the singular very easily; the singular “for he is the child of the kingdom of the devil” would have sounded quite strange. So instead, Joseph went back to such an one and deleted the singular part (an one). In 𝓟 itself, he neglected to restore the original they to “they shall be cast out”, but the 1837 edition set the intended they.
This removal of an one, although grammatically correct, reduces the parallelism of the larger passage. In verses 24 through 31 there are five other occurrences of such an one, but in these cases we have only singular agreement, so there was no need to edit such an one in any of these five cases:
Despite the number disagreement between such an one and they, the critical text will retain the original use of such an one in Alma 5:25, thus maintaining the parallel use of such an one in the larger passage. Moreover, the original text frequently switched the number for generic pronouns. See, for instance, the discussion under 1 Nephi 10:18–19.
Summary: Restore in Alma 5:25 the original use of such an one; even though a generic plural they is later used in the verse to refer to the singular generic such an one, such switches in generic pronoun reference are found elsewhere in the original text.