Here Oliver Cowdery initially wrote a Nephites in 𝓟. He caught his error immediately; but in erasing the final s, he ended up erasing the te as well, giving “it being a day’s journey for a Nephi”. The 1830 typesetter corrected this impossible reading to the correct “it being a day’s journey for a Nephite”. As explained under Alma 23:17, virtually every Book of Mormon name that refers to a people and is derived from a personal name takes the ending -ite (such as Nephite from Nephi ); the only exception appears to be Nehor.
Summary: Maintain in Helaman 4:7 the name Nephite in “it being a day’s journey for a Nephite”; Oliver Cowdery’s correction in 𝓟 of Nephites to Nephi was a scribal error.