What is the validity of a worshiper cutting through the row in the second raka'ah and completing the last raka'ah alone without the row?

What the man did is in accordance with the Sunnah, as he found a place in the row and stood in it. There is nothing wrong with him completing his prayer even if the row is not complete, for the latecomer completes his prayer after the Imam's salutation. And if someone comes and does not find a place in the rows, he stands alone and his prayer is valid.