The original manuscript is not fully extant for “to give his/her light”. The initial h is partially extant, but the following two letters are not. So we cannot be sure whether 𝓞 read his or her. The 1830 edition reads his. In the printer’s manuscript, Oliver Cowdery initially wrote her; then somewhat later with slightly heavier ink flow he crossed out the her and supralinearly inserted his. His correction probably occurred while proofing against 𝓞. Since both the 1830 edition and the corrected reading in 𝓟 read his, 𝓞 undoubtedly did too.
Earlier in the text, there is one other example where his was accidentally changed to her. Interestingly, this passage also refers to the sun being darkened:
Here the 1830 edition accidentally replaced his with her, most likely under the influence of the following use of her when referring to the moon. In the 20th century, the LDS and RLDS editions restored the original his in 2 Nephi 23:10, each time by reference to 𝓟.
Summary: Maintain in Helaman 14:20 the occurrence of his in “the sun shall … refuse to give his light”, just as in 2 Nephi 23:10 where the text uses his to refer to the sun.