Here are some possible sources:
www.1879zuluwar.com (page 3; register and log in) It takes patience to dig through. Has photos of C. B. Lonsdale of Lonsdale's Mounted Rifles & Lonsdale's Mounted Horse. Rupert Lonsdale was Commandant of the 3rd Regiment, Natal Native Contingent. He suffered from sunstroke and was given permission to go back to Isandlhwana.
"The Sun Turned Black: Isandlhwana and Rourke's Drift, 1879" by Ian Knight, 1995. page 141: partial page view"..he had come across a wounded man lying in a donga. [a Boer word for ravine]. Stafford stopped to help him...he said his name was Young, of Lonsdale's NNC--but it took several attempts before he could get his foot in the stirrup and climb up behind. The man had a bad stab wound under his arm, and was so weak from loss of blood that Stafford could hardly feel his grip. When Stafford drove his horse across a donga it faltered for a second climbing out the far..."
Young had lost his horse, and got close enough to the fighting that he got stabbed under the arm by an assegai. We'd have to get the book to read the rest. (found on Google Books)
An officer named Young in Lonsdale's Company of Natal Native Contingent is mentioned in "A Solemn Mockery: The Anglo-Zulu War of 1879: the Myth and the Reality" (2006) Paperback, costs $127 used and we don't have it.
Lonsdale Denoon Young has a profile on Geni.com
This is going to take more work. So far we know there was an officer named Young under Lonsdale's command. Captain Walter Stafford recalled encountering him; they were closely pursued and presumably Young died. See also Daily Mirror, 14 Sep 2017 which has an article about Captain Stafford's medals and quotes the passage above.