Why do many Protestants call their ministers "Reverend," an adjective that means "revered"? On the basis of your friend's argument, shouldn't the only one worthy of reverence be God himself? Why do many Protestants impart on their seminary students the title "Master of Divinity" to denote that this person has completed a step many Protestants deem necessary to being ordained to the clergy? How can any man master the Divine?
I point this out, not to ridicule your friend's deeply-held convictions, but to demonstrate that many Protestants apparently do not think through their objections to Catholic titles of honor for clergy. They too use titles that, if subject to the same semantic scrutiny to which they subject Catholic titles, would have to be rejected on the same grounds.
Catholics call the pope "Holy Father" as a mark of respect because they recognize his call by Christ to image the holiness and Fatherhood of God. The term draws its power from, is dependent upon, and shares in a subordinate manner in the holy Fatherhood of God; it in no way supercedes it or denies the unique holiness and paternity of God alone.
Call No Man "Father"?