Sikhs do call, Guru Nanak as their Prophet. Which is wrong, becos hazrat Muhammad [SAW]was the last prophet, and all others who claim to be one, are hypocrites and false prophet.
Guru Nanak was no Prophet just reformer, it would be clear by reading the fowlling.
Qualities of a Prophet of God
No one can become a prophet of God by his own choosing or effort; Allah has to choose him to be His prophet. And Allah chooses only such persons who have all the qualities necessary for that role, and He does so at the appropriate time in history, and then He protects His prophets from falling into the kind of errors that do not suit a prophet.
The first duty that a prophet of God does is to tell his people that he has been appointed a prophet by God to invite people to His way. Thus, a genuine prophet claims to be a prophet of God, though this claim in itself is not enough. But first and foremost, a prophet has to claim himself to be a prophet of God. The people often question this claim and threaten him to give up his claim. They may even offer him all kinds of temptations that ordinary humans would fall for. But the genuine prophets ignore these and go forward with their God-given mission.
So, the first question you need to ask your Sikh friend is: Did Nanak really claim to be a prophet chosen by God to lead people out of darkness into light?
If the answer is “no,” tell him that this proves that prophethood was conferred on him by his zealous followers and it is meaningless to continue to argue that Nanak was a prophet.
If the answer is “yes,” the second question is: Did Nanak approve of the roles of earlier prophets of God, especially the prophet who came just before him, Muhammad (peace be on him)?
If he says “yes,” ask him then how one can reconcile the fact that Muhammad was the Last Prophet of God and the claim that Nanak was a prophet after the Last Prophet.
The third question is whether all the Sikhs believe that their holy book, called the Adi Granth, was revealed to Nanak by God. If they say “yes,” ask them whether there is any statement to that effect in the book itself and whether Nanak said that the Adi Granth had been fully revealed to him by God. If the answer is in the affirmative, your Sikh friend has to show the proof that the above statements are true, from the Adi Granth itself. This will not be possible for him to do for the following reasons:
1. The Adi Granth is a collection of the writings of many Gurus including Nanak. It was in 1604—Nanak died in 1539—that Arjan Dev, one of the ten Gurus, compiled the hymns of Guru Nanak along with the compositions of both Hindu and Muslim holy men like Jaidev, Surdas, Sheikh Farid and Kabir. The compiled book was enshrined by Arjan in the Golden Temple with the name “Adi Granth.”
2. A prophet of God is a model for all his followers in all aspects of life. But in the case of Sikhism, we find that it was not Guru Nanak, but the tenth Guru, Gobind Singh, who organized the community of Sikhs into a khalsa, “a spiritual brotherhood devoted to purity of thought and action.” He taught his followers to wear long hair (kesh, denoting saintly appearance), underwear (kachha, denoting self-control), an iron bangle (kara, denoting purity in acts), a comb (kangha, denoting cleanliness of mind and body), and a sword (kirpan, denoting fight for a just cause).
3. Towards the end of his life, the aged Nanak returned home to Punjab and settled down at Kartharpur with his family. People came from far and near to hear his hymns and preaching. After his death, his Hindu followers thought him to be a Hindu and his Muslim followers thought him to be a Muslim.
From the foregoing, we understand that Guru Nanak was not a prophet of God, but a religious reformer of his times.