The Septuagint is the Greek translation of the Hebrew Tanakh made ~2 centuries before Christ--the translation was decidedly not made by Christians.
The earliest believers in Jesus in Judea & Galilee were Jews--in fact, it is very likely that every book of the New Testament except Luke & Acts was written by a Jew. That the overlap between Judaism & Christianity lasted for several decades is evident in the words of a number of writers (e.g. Acts 18:2 refers to a man who was decidedly Christian as a Jew).
The full separation of Judaism & Christianity into distinct religions did not happen until the Flavian era (70s-90s), and was closely tied to the destruction of Jerusalem.
So not only the Greek translation of the Old Testament, but a very substantial portion of the New Testament, were written by Jews. Because Greek was the lingua franca of the eastern Roman Empire, much of the early Christian preaching & record-keeping happened in Greek, and when quoting the Old Testament in a Greek text, the Greek Septuagint is usually used.
Your friend may be referring to Origen's edits to the Septuagint in the 3rd century--he made an effort to correct/harmonize variations in the text...but this was well after the New Testament was written.
Claims that a religious group corrupted an important text are common -- Christians also accused the Jews of editing the Hebrew Tanakh to remove some of the Messianic prophecies.
We don't have the autographical (original) texts for either the Hebrew Tanakh or the Greek Septuagint, and there are known disagreements between them. For a deeper dive on textual criticism, see my thoughts here.
Sources
https://www.britannica.com/topic/Septuagint
https://ccel.org/ccel/edmundson/church/church.xi.html
https://www.newadvent.org/fathers/01286.htm chapter 72
