Theophylact of Ohrid

Pilate's sending of a subordinate to Herod appeared as the beginning of friendship, since Pilate does not appropriate for himself the prerogatives of Herod. Moreover, observe everywhere how the devil, in order only to prepare the death of Christ, brings together what stood apart from one another, instills harmony and friendship between those who were enemies. Is it not a shame for us that he, in order to put Christ to death, even reconciled enemies, while we, for our own salvation, do not even keep friends in friendship with us?

Commentary on Luke ↗

Ambrose of Milan

In the figure of Herod and Pilate also, who became friends from enemies through Jesus Christ, the figure of the people of Israel and the Gentile people is preserved, so that through the passion of the Lord there will be future harmony between them both: in such a way, however, that the pious people of the nations receive the word of God and transmit their faith to the Jewish people with their devotion; so that they too may clothe the body of Christ with the glory of their majesty, whom they had previously despised.

Exposition Of The Gospel Of Luke 10.103 ↗

Bede

And Herod and Pilate became friends on that day. For previously they had been enemies to each other. This most nefarious pact between Herod and Pilate, which they made in killing Christ, is still maintained by their successors as if by hereditary right, when pagans and Jews, although differing in race and religion as much as in mind, nevertheless agree in persecuting Christians, and in trying to destroy faith in Christ within them.

On the Gospel of Luke ↗

Bonaventure

Because Herod did not find Christ worthy of death, therefore he himself also refused judgment; and so it is added: And he sent him back to Pilate, as to a friend. On account of which he adds: And Herod and Pilate became friends from that day; for before they had been enemies to one another. …

Commentary on Luke, Chapter 23 ↗