Good for you that you want to stick up for your friend.
You actually have two acts here, one by the bouncer, and one by the second man. The bouncer works for the bar, and he was acting within his job, so the bar is responsible, as his employer, for what he does. And what he did definitely violated federal (and probably state) law against discrimination in "places of public accomodation." Even if they have some sort of defensible rule against headwear in the bar, they applied it discriminatorily against your friend, because they let the other guy in with a baseball cap.
If the second guy who spoke to him also works for the bar, then it's the same for him. But if he is just some jerk, and he didn't actually threaten your friend, then -- well, then, he's just a jerk.
Your friend could complain to the Equal Employment Opportunity Commission or the Florida antidiscrimination office, but I would first suggest talking to the bar owner or manager. He or she should be grateful to get information that an employee is antagonizing customers and also opening the bar up for legal liability. If the owner doesn't care -- well, the owner should care, and I can't think of a better way to see to that than to file a complaint or even just have a lawyer sending him/her a polite letter describing what happened and asking what s/he intends to do about it. That usually gets their attention.
Although you were not directly affected, you might consider also writing a brief note to the bar telling them that this incident disturbed you and will deter you and your friends from coming back.
Good luck!
-- Susan Gellman
