The 1830 typesetter skipped the word one, perhaps accidentally or possibly because he considered it unnecessary or awkward. Nonetheless, there is no overwhelming grammatical reason for deleting one; it seems to mean ‘someone’ in this context.
Related to this use of one is a nearby correction that Oliver Cowdery made in the printer’s manuscript:
Oliver’s correction here appears to be immediate. This additional one in 2 Nephi 3:18 appears to be the result of a visual copying error. Oliver was copying from the original manuscript, which would have had “& I will make” in two contiguous lines (verses 17 and 18). When he started copying the second “& I will make”, Oliver’s eye strayed up one line so that he started to write “& I will make one a spokesman for him” when he should have written “& I will make for him a spokesman”. He caught his error, deleted the one, supralinearly wrote the for, and then continued the rest of the sentence inline. This visual error argues that the word one was indeed in the previous line of the original manuscript and that the 1830 typesetter made an error when he deleted it.
Summary: The original manuscript probably read “and I will make one a spokesman for him”; the 1830 typesetter omitted the one, perhaps accidentally.