As far as the movie goes, he's a man.
All for a box of chocolates

Moore employed a similar tactic for the Joker in Batman: The Killing Joke (my favourite Moore of all).
All for a box of chocolates

I thought the movie made clear that V was a man.
The film does make it clear. The ambiguity comes from the graphic novel.
I've lived upon the edge of chance for 20 years or more
Del Rio's Song

I thought the movie made clear that V was a man.
He begins the film by telling Evey that he is "a man in a mask".
In the diary that talks about his origin, the doctor refers to him as "the man in room 5"
And everyone in the movie, when speaking of V says "he", "him" and "his".
~Brad

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You friend is very observant. That is another plot point this dreadful movie decided to throw out from the novel. The whole point of V is that you don't know who he (or she) is. Alan Moore's fantastic graphic novel showed that there was the possibility that all of V's backstory (survivor of horrendous medical experimentation at the concentration camp) may have all been made up by V. So the reader is left all the way through guessing as to the truth of V, you don't know his (or her's) name, you think you know his (or her's) backstory but you can't be sure, and you think that he (or she) is male but can't be sure. None of those questions are answered by the end of the novel.