Can a person not engaged in prayer be corrected by someone who is praying?
Chapter on the Two Prostrations of Forgetfulness
Al-Mughni
Book of Prayer
Primary text
It is permissible for someone who is not praying alongside the person who is praying to correct the latter's recitation.
Supporting text
Evidence for this is found in a narration by Al-Najjad, who reported that while he was sitting in Mecca, a man was praying near the Maqam (station of Abraham), and another man sitting behind him was prompting him. That man prompting was Uthman, may Allah be pleased with him.