Here the 1858 Wright edition accidentally replaced the singular man with the plural men. The 1908 RLDS edition restored the original singular to the RLDS text. Elsewhere in the text, there are four occurrences of “whereby man can be saved” but none of “whereby men can be saved”. And one of these other occurrences with man is in king Benjamin’s actual speech, which is referred to here in Helaman 5:9:
So the use of the singular here in Helaman 5:9 is consistent with king Benjamin’s actual words in Mosiah 4:8.
Summary: Maintain in Helaman 5:9 the use of the singular man in the phrase “whereby man can be saved”, which is consistent with Benjamin’s own words in Mosiah 4:8.