Brother lets first analyze what does the word Gaib means…?

Gaib means Unseen… Things which are not seen to a Normal Human being..then now what is the Defination of a NABI..Nabi is a person who gives knowledge of the Unseen..Gaib ki khabar dene waaale ko Nabi kehte hai..If your friend is saying that RasoolAllah  sal-lal-lahu alai hi wa sallam was a Nabi and he is doubting that whether he has knowledge of the unseen..It like he isintroducing you to someone he calls a Doctor and then asks i doubt whether he can Read and Write