Is it permissible for someone who enters the mosque after the Imam has said the Taslim and finds a latecomer completing his prayer to stand beside him to make him an Imam so that he may attain the reward of congregational prayer, and is his prayer with him valid?

If a latecomer enters the mosque and finds someone praying in congregation or alone, it is permissible for him to pray with him to his right, with the intention of leading the prayer for the one who is praying, in order to attain the virtue of congregational prayer. After the latecomer or the individual finishes his prayer with taslim, this newcomer stands up and completes his prayer. This is indicated by the general proofs of the virtue of congregational prayer, and the hadith: "Will not a man give charity to this one by praying with him?"