Praise be to Allah.
First of all, it is essential to understand that guidance to the true religion of Islam and belief in the Prophet (peace and blessings of Allaah be upon him) and the message he brought comes about only by the help of Allaah, and that we are required to explain this and produce evidence of it.
There is a great deal of clear evidence for the one who seeks to find out the truth about the Prophethood of Muhammad (peace and blessings of Allaah be upon him), including the following:
Allaah chose him even though he had grown up as an orphan and was illiterate, knowing neither how to read or write. All good qualities and virtues were perfected in him, to the point of ultimate perfection. All these good qualities were combined and firmly established in him, something which no one else can attain except the Prophets whom Allaah protected and guided. This combination of perfect qualities is one of the greatest proofs of the truth of his Prophethood (peace and blessings of Allaah be upon him).
The revelation of Allaah was bestowed upon the Prophet (peace and blessings of Allaah be upon him). The Angel of Revelation, Jibreel (Gabriel), who is recognized by all the People of the Book [Jews and Christians], came to him, and when he received revelation, there were unusual effects on him that indicated the intensity of the effort involved in receiving the Message of Allaah from Jibreel, upon whom be peace. His Companions witnessed this when he received Revelation, and they believed that the revelation was something that came from an outside source and was beyond his control; it came to him from (Allaah), the Wise and Praiseworthy.
Allaah supported the Prophet (peace and blessings of Allaah be upon him) with physical miracles, with which he challenged his people. Among the most important of these were the splitting of the moon and the Night Journey to Bayt al-Maqdis (Jerusalem). They were unable to match these miracles, and so they were a decisive, divine testimony to the truth of his Prophethood (peace and blessings of Allaah be upon him).
Allaah supported him with miracles and evidence for people of reason and understanding, the greatest of which was the Qur’aan. This was the greatest sign, which contains in its pages miracles of both style and content, which all of mankind cannot match or imitate, even if they gather together for that purpose. This is clear from the statements of the most eloquent of the Arabs, who were completely unable to match it. The Qur’aan was – and centuries later still is, praise be to Allaah – an eternal miracle as is evident in its brilliant style, its information about the Unseen, its fulfilment of the promises of Allaah, the superiority of its laws, and the fact that its rulings are appropriate for every time and place – in addition to the fact that it has such a moving impact on people’s hearts.
The Qur’aan refers to the signs and proofs surrounding the person of the Prophet (peace and blessings of Allaah be upon him), which testify to his truthfulness. For example, he was free of sin and trivial spitefulness; he grew up illiterate in the Arabian Peninsula, and did not receive any religious knowledge from beyond its borders, nor did he spend time acquiring any religious knowledge. When he reached the age of forty, he brought divine knowledge that none of the earlier or later generations could produce. He also received special protection and care from Allaah, which kept him safe at times of crisis; if it were not for this protection he would most certainly have been doomed to destruction. None of this could have happened except to one who was truly sent from Allaah.
His coming was foretold in the Tawraat (Torah) and Injeel (Gospel), and some of the People of the Book saw that these prophecies applied in totality to the Prophet (peace and blessings of Allaah be upon him), which led them to embrace Islam. There are still passages in the Bible, in both the Old and New Testaments, in the versions that are accepted by the People of the Book, that clearly refer to the Prophethood of Muhammad (peace and blessings of Allaah be upon him) – for example in the Gospel of Barnabas, which is considered by Christians to be the most authentic Gospel.
The Message of the Prophet (peace and blessings of Allaah be upon him) was not unique to him, but was in nature and content similar to the message brought by other Messengers before him. Allaah had sent Prophets and Messengers, such as Moosa (Moses), ‘Eesa (Jesus) and others, to the Children of Israel, and great numbers had believed in them and borne witness to the truth of their Books, which were similar in general terms to the message brought by the Qur’aan. This was eloquent testimony to the truth of the Message with which he was sent, especially as it belonged to the same type of Message to whose truth they had testified.
When Allaah sent Muhammad (peace and blessings of Allaah be upon him) with the same Message as the Prophets who had come before him, the Qur’aan came to confirm their Books and their Prophethood, and to call people to believe in them. So when the People of the Book disbelieved in him and his Book, it meant that they were disbelieving in their own Books and Messengers. As the Qur’aan contained the same principles as their books, and confirmed them, this meant that it was the least likely to be fabricated or to have come from a source other than Allaah, because all of them came from Allaah, may He be exalted.
In summary, then, the proof of truthfulness is quite clear in this case. The evidence is there both in (historical) reports and on the grounds of common sense. This will cure the ill (i.e., the one who has doubts) and quench the thirst (of the one who longs for knowledge), as Allaah says (interpretation of the meaning): “… Indeed, there has come to you from Allaah a Light (prophet Muhammad (peace and blessings of Allaah be upon him)) and a plain Book (the Qur’aan), wherewith Allaah guides all those who seek His Good Pleasure to ways of peace, and He brings them out of darkness by His Will unto light, and guides them to a Straight Way” [al-Maa’idah 5:15-16].
In order to provide the Christian mentioned in the question with proof and evidence, it should be sufficient to give him a translation of the meanings of the Qur’aan. If he believes, then praise be to Allaah, and if he turns away, then destroy not yourself in sorrow for him (cf. Faatir 35:8). Allaah leaves astray whom He will and guides whom He will, and He is the Most Knowing of Guides.