See how some Christians know not only their books, but go into the tafseers to see what they can misinterpret, to give us da'wah, whereas as Muslims, da'wah is our duty, and we don't even know our own book let alone anyone elses.
The Qur'an is clear. Allah says:
And [I] will make him ['Iesa (Jesus)] a Messenger to the Children of Israel
(saying): "I have come to you with a sign from your Lord, that I design for you out of clay, as it were, the figure of a bird, and breathe into it, and it becomes a bird by Allah's Leave; and I heal him who was born blind, and the leper, and I bring the dead to life by Allah's Leave. And I inform you of what you eat, and what you store in your houses. Surely, therein is a sign for you, if you believe. (3:49)
Your friend only needs to read two ayaat later after the verse he was looking up to find:
And [mention] when Jesus, the son of Mary, said, "O children of Israel, indeed I am the messenger of Allah to you
confirming what came before me of the Torah and bringing good tidings of a messenger to come after me, whose name is Ahmad." But when he came to them with clear evidences, they said, "This is obvious magic." (61:6)
Jesus (peace be upon him) also says in the Bible:
He answered, “I was sent only to the lost sheep of the house of Israel
.” (Matthew 15:24)
So both books agree clearly that Jesus (peace be upon him) was sent only to the Children of Israel. In the Qur'an, Allah is telling us, and no word is truer than His.
Tafsir Ibn Katheer gives no reference for the statement about Greeks being invited to Islam. It says that the disciples were sent to various areas of ash-shaam. Ash-Shaam is not the land of the Greeks. However, there were some Greek speaking Israelites around at the time. Maybe there were some Greeks that had travelled there, who would hear the message being conveyed to the Israelites. Perhaps this is what is meant. All we can say is Allah knows best. But this does not change or alter the fact that Jesus (peace be upon him) was sent to the Children of Israel, nor does it suggest that he went contrary to his mission (na'oothu billah).
Maududi gives this insight on Greek speaking Jews/Christians:
[...] the language spoken by the Prophet Jesus and his contemporary Palestinians was a dialect of the Aramaic language, called Syriac. More than 200 years before the birth of Jesus when the Seleucides came to power Hebrew bad become extinct in this territory and been replaced by Syriac. Although under the influence of the Seleucide and then the Roman empires, Greek also had reached this area, it remained confined only to that class of the people, who after having access to the higher government circles, or in order to seek access to them, had become deeply Hellenized.
It is also worth noting this, about how Christianity began to be preached to non-Israelite communities;
The earliest followers of the Prophet Jesus (peace be on him) believed him only as a Prophet, followed the Mosaic Law, did not detach themselves from the other Israelites in the matter of beliefs, religious in junctions and rites of worship, and differed from the Jews only in that they had affirmed faith in the Prophet Jesus as the Messiah (the Christ) whereas the Jews had refused to acknowledge him as the Messiah. Later, when St. Paul entered this community, he started preaching and proclaiming this religion also to the Romans and the Greeks as well as other non-Jewish and non-Israelite communities.
For this purpose he invented an altogether new religion, the beliefs, doctrines and injunctions of which were entirely different from those taught by the Prophet Jesus. This man had never met the Prophet Jesus but was a bitter enemy of him during his lifetime, and even remained an enemy of his followers for several years after him. Then, when he entered his community and started inventing a new religion, he did not cite any saying of Jesus as his authority but based everything on his own inspiration. The only object before him while framing the new religion was to make it acceptable to the gentile world. He declared that a Christian was free from all restrictions of the Jewish Law. He abolished all restrictions of the lawful and the unlawful in food. He repudiated the circumcision, which was particularly disagreeable to the non-Jewish world. So much so that he innovated the doctrine of the divinity of Christ and his being son of God and the Atonement for the original sin of mankind by dying on the Cross, for it immensely suited the polytheistic taste of the common man. The early followers of Christ resisted these innovations, but the flood gate opened by St. Paul caused a huge multitude of the non-Jewish Christians to enter this religion against whom the handful of true Christians could not stand for long. However, until about the end of the 3rd century A.D. there were still many people who denied the divinity of Christ. But in the beginning of the 4th century (in 325 A.D.) the Nicaea Council recognized the Pauline doctrines and admitted them as the basis of Christianity. Then the Roman Empire itself turned Christian and in the reign of Emperor Theodosius, Christianity became the state religion. Consequently, all the books that clashed with this doctrine were declared as heretical, and only those books were held as canonical which agreed with it...
Source: the meaning of the Qur'an, by S. A Maududi, commentary to Surah as-Saff, Surah 61
As to Islam being only for the Children of Isra'eel, no, it is for the whole world. However, each messenger, prior to Muhammad :saws: , was sent to their respective nations, and for a specific time period. However, Allah knew with His foreknowledge, that those religions messages would get distorted by people, and would not remain the original message sent, thus a new messenger would be sent. It was with His wisdom, that the final messenger Muhammad :saws: was sent to all the world, and his message is the only one to remain undistorted and in tact, as Allah has promised to preserve it, thus no need for any new messenger. So Islam is indeed sent for all the world, not any distortions.
And Allah knows best.