Rules around bosses shouting at you?

I've a friend who was recently roared at badly to the point of tears by her boss. Now as far as I've been able to pick up she was definitely in the wrong herself but the roaring she got was apparently horrendous and it isn't the first time she's been ate alive, including times where she's done nothing wrong.

What are the rules on this? Surely bosses aren't allowed to scream at people?