Praise be to Allah.
It seems that your friend, in his ignorance of the true nature of Prophethood, does not make a distinction between a Prophet and an astrologer, and so he compares the Prophethood of Muhammad (peace and blessings of Allaah be upon him) and the astrology of the French physicist Nostradamus who lived in the 16th century CE and composed books of fantasies and illusions. His books are written in ambiguous language about events that will happen up until the end of the world, which he expected would happen in the year 3797. A person like this needs to understand the true nature of Prophethood and why mankind needs to believe in it, and he needs to know that the Unseen can only be known through the Prophets who were chosen by Allaah. He needs to know that Prophethood is a gift from Allaah, may He be glorified, and it cannot be attained by a person’s desires or own efforts. Prophethood is a pure blessing from Allaah, as is indicated in His words (interpretation of the meanings):
“Those were they unto whom Allaah bestowed His Grace from along the Prophets, of the offspring of Adam, and of those whom We carried (in the ship) with Nooh…” [Maryam 19:58]
“Thus will your Lord choose you…” [Yoosuf 12:6]
“… I have chosen you above men by My Messages and by My speaking (to you)…” [al-A’raaf 7:144]
“… Allaah knows best with whom to place His Message…” [al-An’aam 6:124]
Then he should realize that the way in which Allaah conveys His message to His Prophets and Messengers is by wahy (revelation). Allaah says (interpretation of the meaning): “Verily, We have inspired you (O Muhammad) as We inspired Nooh and the Prophets after him…” [al-Nisa’ 4:163]. There is nothing wrong with explaining to him the three types or levels of wahy so that he may have a better understanding of this channel of communication between Allaah and His creation, which is so essential for mankind. These three types or levels have been mentioned by Allaah in the Qur’aan (interpretation of the meaning): “It is not given to any human being that Allaah should speak to him unless (it be) by Inspiration, or from behind a veil, or (that) He sends a Messenger to reveal what He wills by His leave. Verily, He is Most High, Most Wise.” [al-Shooraa 42:51]
These three types or levels may be further explained as follows:
Inspiring something in the heart of a Prophet in such a way that he has no doubt about it and is certain that it is from Allaah. It was reported in a hadeeth that the Prophet (peace and blessings of Allaah be upon him) said: “The holy spirit (i.e., Jibreel) blew into my heart that no soul dies until its provision (rizq) and appointed time have expired, so fear Allaah and be polite and reasonable when you seek to earn a living from others.” (Reported by Ibn Hibbaan). This type of revelation also includes the visions of the Prophets, which are true, as is proven in the hadeeth of ‘Aa’ishah Umm al-Mu’mineen (may Allaah be pleased with her) who said: “The first thing that happened to the Messenger of Allaah (peace and blessings of Allaah be upon him) of wahy was true vision in his dreams: he did not see something but it would become as true as daybreak.” (Agreed upon).
Therefore the Prophet and Friend of Allaah, Ibraaheem, hastened to sacrifice his son when he saw that he was commanded to do so in a dream. He took this dream as being a revelation to him, as Allaah tells us (interpretation of the meaning):
“So We gave him the glad tidings of a forbearing boy.
And, when he (his son) was old enough to walk with him, he said, ‘O my son! I have seen in a dream that I am slaughtering you (offering you in sacrifice to Allaah), so look what you think!’ He said: ‘O my father! Do that which you are commanded, in sha Allaah (if Allaah will) you will find me of al-saabireen (the patient ones).’
Then, when they had both submitted themselves (to the Will of Allaah), and he had laid him prostrate on his forehead (or on the side of his forehead, for slaughtering),
And We called out to him: ‘O Ibraaheem!
You have fulfilled the dream (vision)! Verily, thus do We reward the muhsinoon (good-doers).
Verily, that was a manifest trial,
And We ransomed him with a great sacrifice (a ram).” [al-Saaffaat 37:101-107]
Speaking to His Messengers from behind a barrier. Allaah spoke thus to Moosa, upon whom be peace, as He says (interpretation of the meanings):
“And when Moosa came at the time and place appointed by Us, and his Lord spoke to him…” [al-A’raaf 7:143]
“And when he came to it (the fire), he was called by name, ‘O Moosa!
Verily, I am your Lord! So take off your shoes, you are in the sacred valley, Tuwa.
And I have chosen you. So listen to that which is inspired to you.
Verily, I am Allaah! None has the right to be worshipped but I, so worship Me, and perform al-salaat (prayer) for My Remembrance.’”[Ta-Ha 20:12-14]
Revelation to a Prophet via an angel. This is what Allaah refers to in the aayah (interpretation of the meaning): “… or He sends a Messenger to reveal what He wills by His leave…” [al-Shoora 42:51]. This messenger is Jibreel, upon whom be peace.
The angel came to the Prophet in three forms as is proven in the shar’i texts. These three forms are:
The Messenger (peace and blessings of Allaah be upon him) saw him in the form in which Allaah created him. This happened to the Prophet (peace and blessings of Allaah be upon him) twice.
The revelation came with a noise like the sound of a bell, which went away after the Prophet (peace and blessings of Allaah be upon him) had understood what he had said.
The angel appeared in the form of a man and spoke to him, and the Prophet (peace and blessings of Allaah be upon him) understood what he said. This was the easiest of the three ways for the Prophet (peace and blessings of Allaah be upon him). This what happened the first time Jibreel came to the Prophet (peace and blessings of Allaah be upon him) in the Cave of Hira’.
Then you should tell your friend something of the evidence of the Prophethood of Muhammad (peace and blessings of Allaah be upon him) and his miracles (see Question # 2114), the chief of which is the Qur’aan. There is no harm in giving him a copy of the translation of its meanings. If he believes, then praise be to Allaah, and if he remains a disbeliever after this, then he will bear the consequences of his kufr and will be one of the losers, so do not worry about him, for Allaah is independent of all creatures.