Here in the printer’s manuscript, Oliver Cowdery originally wrote fathers as father; but virtually immediately Oliver inserted inline the plural s (and without any change in the level of ink flow). Here we are talking about the tradition of the Lamanites, and clearly they had more than one founding father: one would count Laman, Lemuel, and the sons of Ishmael as their fathers. But the same plurality also holds for the tradition(s) of the Nephite fathers, where not only Nephi but also Jacob and Joseph (and probably Zoram) count as founding fathers. (See the listing of the Nephite and Lamanite tribes under Jacob 1:13.) Overall the text has instances of only the plural fathers in the expression “the tradition(s) of one’s father(s)”. In all, there are 28 instances, of which 18 refer to the Lamanite fathers and 10 to the Nephite fathers.
Summary: Maintain the plural fathers in Helaman 5:51 (“the tradition of their fathers”); the plural fathers always occurs in this expression.