As discussed under 1 Nephi 22:4, there are quite a few cases in the manuscripts where Oliver Cowdery mixed up the two words whether and whither. Here in 𝓟 (𝓞 is not extant), Oliver wrote whither, perhaps under the influence of the semantically related where at the beginning of the sentence. John Gilbert, the 1830 typesetter, emended 𝓟 by overwriting the i of whither with an e, thus giving the obviously correct whether (which is also the reading of all the printed editions).
Summary: Maintain the use of whether in Helaman 9:12; in the printer’s manuscript, Oliver Cowdery wrote whither (meaning ‘where to’), an impossible reading in this context.