You forgot to hang up, I heard your conversation

So this just happened in my office. My colleague placed a call to our boss to discuss an email she received from him. My boss questioned her competence and bashed her over the phone. She forgot to hang up and then seriously bashed our boss, questioning his competence and made silly remarks about him to us in the office. Moments later, she gets a message on her phone from the boss saying "You forgot to hang up, I heard your conversation”. My colleague is now worried. Please what will you do if this happened to you?