What is the ruling on the prayer of one who nudged another person so that he would step back to pray with him while the Imam was leading the congregational prayer, and that person moved back more than two steps? And is the prayer of the nudged person invalidated due to his stepping back or not?

The position of a single congregant is to the right of the Imam, and the position of two or more congregants is behind him.

If a worshipper finds an Imam and one congregant already present, he should gently signal the congregant to move back with him so that they both stand behind the Imam. In this situation, the prayer is valid.