OVI,
I am a member of the commemorative 5th USCI, Co. C, based in Toledo, Ohio. In 2014, several members participated in the 150th Anniversary of New Market Heights. I've looked up his service record, and he would have been at New Market Heights. He was recruited by Milton Holland, was in hospital at New Berne, North Carolina, and eventually got full pay.
Your friend would have been part of a grand ceremony recognizing USCT descendants who had fought there. We were also given the privilege to access private and county property where the men of the 5th marched and camped. I personally took a stone from a creek they crossed.
That said, the members of my organization like to connect with descendants of members of the 5th so we can honor them and glean from their family histories. Can you send me a private message and connect me with your friend.
Thanks,
-Yulie
EDITED: Retraction: I can't say for certain if Walker was at the Battle of New Market Heights in September 1864. He was detached as a regimental pioneer in May 1864, May-Nov 1864 and July-Aug 1865. Specifics are not on the muster cards. It's for certain that the 5th was in the vicinity of Richmond, therefore it's a possible maybe might guess that he was working with the Army Corp of Engineers thereabouts.