In the 1849 LDS edition, the typesetter accidentally dropped the perfect auxiliary had, which created the dialectal past-tense expression “they done”. This nonstandard use of done for did can be found ten times in the original text (such as “this he done” in Jacob 7:2), and in one of these cases (in Ether 9:29) the nonstandard done was retained for some time after Joseph Smith’s editing of the text for the 1837 and 1840 editions. Thus it is not surprising that here in 1 Nephi 7:20 the subsequent 1852 and 1879 LDS editions continued using “they done”. For further discussion of the simple past-tense form done, see under past tense in volume 3.
Summary: Based on the earliest textual sources, the standard “they had done” for 1 Nephi 7:20 should be maintained instead of the dialectal “they done” that was accidentally introduced into the 1849 edition.