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