Some of the brothers make themselves a sutrah for the one who came late to the prayer. (This is) in terms (of the fact) that they sit in front of him, believing that there is reward in that. So what is the ruling on it?
As for him being a sutrah, then there is no problem in that. As for the one sitting as a sutrah, will he will have a reward or not? He will have a reward because he is doing good to him [the one praying]. And Allah knows best. Yes. Verily Actions depend on intentions.
[Sharh Ibn Majah no. 74]
Faisal Ibn Abdul Qaadir Ibn Hassan