Exegesis:

egenonto de philoi … met’ allēlōn ‘(they) became friends with one another.’ met’ allēlōn, which is virtually redundant semantically, is best understood as strengthening philoi.

en autē tē hēmera ‘that very day.’

proupērchon gar en echthra ontes pros hautous ‘for previously they were at enmity between themselves.’

prouparchō ‘to exist before,’ here virtually an auxiliary verb denoting that the event or situation to which the following participle refers, happened in the past.

echthra ‘enmity.’

Translation:

Became friends with each other, or, ‘became friends,’ or, ‘they came to like (or, to be on good terms with) each other.’

For introduces explanatory matter, not cause or reason, and must therefore not be rendered literally, cf. “… became friends; they had been enemies before this” (Good News Translation), “Herod and Pilate, who had been at enmity, …” (The Four Gospels – a New Translation), H. et P. devinrent amis, d’ennemis qu’ils étaient auparavant (Bible de Jérusalem), ‘whereas before the two had been each other’s enemy’ (Bahasa Indonesia).

They had been at enmity with each other, or, ‘they had been enemies’ (cf. on 1.71), ‘they had not liked each other,’ etc., the opposite of the preceding clause. Both ‘to be friends’ and ‘to be enemies’ indicate a reciprocal relationship; hence they are often rendered by reciprocal verbal forms, e.g. in Balinese, Toraja-Sa’dan.

Quoted with permission from Reiling, J. and Swellengrebel, J.L. A Handbook on the Gospel of Luke. (UBS Handbook Series). New York: UBS, 1971. For this and other handbooks for translators see here . Make sure to also consult the Handbook on the Gospel of Mark for parallel or similar verses.