Robin, the current Sacred Harp gives 1st date of publication right on the page, which will be a big help to avoid later music.

Tell your friend to also get hold of the reprint of the The Missouri Harmony. It was first published in 1820 & most of what's in it was in the earlier Kentucky Harmony. Everything that was added later is in a separate supplement, so anything in the main body of the book should be OK. Lots of the tunes are also in Southern Harmony & Sacred Harp, but it has more 4 part harmonies. There are also some wonderful minor tunes that are not in the Sacred Harp.