I have a friend who got sober in 2007. Went to AA meetings and worked the program. Lived in a sober living situation. About two or three years later, he was having trouble sleeping and was prescribed Ambien. He took it, and in a sleep- walking state- unplanned- went to the store, bought a 12 pack, took it home and drank all of it. He has not had a drink since, but still reset his sobriety date.
In the case of your friend, I would say there is no need to reset the date- as long as that one drink remains just that- one drink. I'm still not sure why your friend felt the need to do that, since it could have very easily reawakened things and led to a binge- but he's free to make his own choice. I doubt most of us on here would be able to do that without being sucked down the drain into active alcoholism again.