When was it decided that one person of the Trinity would become the Redeemer?
It was decided in eternity past, before anything was created, when the Father, Son, and Holy Spirit came together in determinate counsel. Knowing humanity would fall in Adam, the triune God predetermined that one person of that triune God would have to be the Redeemer.
Les teaches that way back in eternity past, the Father, Son, and Holy Spirit came together and by a determinate agreeing of their three minds decided to create the universe and place the human race on planet earth. They knew in foreknowledge that the human race would fall in the man Adam, and in that predeterminate counsel they knew one person of the triune God would have to be a Redeemer. This decision belonged to the predetermined counsel of Acts chapter 2 and stood before anything was ever created, moving the whole plan forward.