Is what the Second Caliph said — that the Prophet's (peace be upon him) being bewitched was an act of man and not an act of God, and that God decreed that the Prophet be bewitched, and that this attributes evil to God — correct?

Your other friend is correct in saying: "Allah decreed that the Prophet, peace and blessings be upon him, be bewitched" or "Allah tested His Prophet, peace and blessings be upon him, with magic." For magic is not attributed to Allah Almighty as His own act; rather, it is among His creations and effects, which contain both good and evil. All of Allah's actions are good.