Do you believe that what your co-worker is doing is okay? If you don't then you need to explain this to your co-worker, that you BELIEVE that his behavior is not okay and give specifics why.
You need to speak to what you BELIEVE and not attack him for being a bad person. If he is your friend and sees that you don't like his behavior than he might choose to stop the bullying.
And if he doesn't, perhaps you should reconsider the friendship.
Good luck! Mitzi