Seems to me you've waited too long and now you've crossed the line of no return, meaning "I only see you as a friend, bla, bla, bla bullshit". Now, there's no point in them two hooking it up obviously, so you go to your mate and call up male solidarity and all that crap, tell him she's yours and you want him to back down. Then you will be miserable for not telling her sooner, he will be miserable because he's aching for a girl he can't have and most of all she will be miserable wondering why that guy never called her again (she'll probably think she's too fat). There, works out for everyone.