Well, if there is an opportunity in conversation where you feel comfortable telling her you think it might be a boy, then do it. Don't just say outright that it is a cockerel, because then it becomes a challenge, and she may give you a bad reaction. Just suggest it. And if she insists it is a female, then drop it.
Often people need to figure these things out for themselves. She's gonna know in a few weeks when it crows, anyway, so you're not doing any harm by keeping what you know to yourself. I guess I'm saying that you should follow your intuition based on what you know of your friend's personality and her past behavior.