Becoming a witness of an unjust or rude situation, one often is at a loss of words.
Here some suggestions for interference:
A) “where are your manners?”
B) “what’s all this here?”
If the response is” it’s none of your business” you may answer:
A) “your behaviour is offensive and makes me feel uncomfortable”
B) “I’m feeling insulted as a human being”
now hope you don’t get hit in the nose