Sometimes a friend is just a friend, but if your girlfriend speaks of one new male friend more than any other, it may be that she is having an affair with him.
CatchingThemCheat.com notes that even if this friendship is something she admits to openly, it is still a relationship of which you should be wary.
You should be particularly suspicious if you never met this friend. AskMen.com warns you to be concerned if she suddenly never mentions the friend again after talking about him constantly.