Your Protestant friend is depending mightily upon his own opinions about important things that are best left to people who know an awful lot more than he could ever hope to know.
I’ve noticed this about Protestants. They’ve got this urge to “understand” everything before they can submit to it. The problem is that they can never really understand much of anything. They submit to the Trinity without being able to comprehend Trinity, and fail to give credit to the Fathers who defined it for us many centuries ago. What makes them think they understand Trinity? No one else does! So why do they need to understand anything else the Church teaches? Shouldn’t it be enough to recognize the logic that Jesus established a visible, authoritative Church, led by Apostles and their successors, guided by the Holy Spirit and made infallible unto all Truth? Submit to those teachings, and learn from them, or forever be spinning around trying to understand the incomprehensible!
lol But that’s okay. Thanks.