From the earliest Church that has been held; in the New Testament it is referred to by the words “laying on of hands”.
It took a while for things to sort themselves out, and too often it is our presumption, often simply lying below the surface of our thinking, that the sacraments pretty much looked like what we see and have today. It is perhaps not quite that simple.
Christ had a number of followers - remember that He sent out 70 to preach the Good News. Not just the Apostles, but a bunch more.
However, His focus was on the Apostles, and it is clear that He singled them out from all of the rest of His followers, both in teaching and in the power and authority He passed on to them.
After he was gone from them, and some time later, they started to pass on that authority and work that He had intrusted to them. From the beginning, the Apostles selected others to do what they did - that is, to oversee the spread of the Church. They also appointed others to assist them, and from that we get the presbyters (whom we now primarily call priests) and deacons.
And from that, the deacons were to assist, but did not say Mass or “lay on hands”, and it would appear that the presbyters said Mass, but did not “lay on hands”.
Further, if one goes back to the Old Testament, their worship services were conducted by a few chosen men, and while all offered sacrifice, the leaders of the religious services werre restricted to the few chosen to be their religious (liturgical, that is, worship) leaders. So the pattern was not completely unknown to the Apostles, and by the separation off of the Apostles from the disciples, Christ was making it clear that He was setting them aside from the others to do what He commanded.
It may be possible that others than the Apostles were at the Last Supper, but if you read the accounts, no one else shows in the writings. Likewise the scene where they are commanded to go forth, after the Resurrection.
Both in the Old and New Testaments, people other than those set aside for leading worship were able to offer sacrifice, but in both, there was a clear delineation that the priesthood had a function in worship and offering sacrifice that was of a higher order.
The matter is treated in the Catechism of the Catholic Church, not all in one spot, and perhaps not in a way that you can use for your friend. However, it is how the Church has acted since its beginning. And that is where, with some (or many) of our Protestant brethren, we run into difficulties, because not all the Church teaches is specifically set out in Scripture; we also rely on Sacred Tradition.