What should be done ?

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My friend asked me this. A professor from his college rang up to a girl student's house and spoke ill of her, none of which was true. He tried to spoil her name at college and at her home. Thank God, her parents trusted her and tried to report about the worse behavior of the Professor to the College. But they don't think he will be punished. My friend says that, a group of students have decided to attack him in his house, so that he does not do the same to any on else. Please tell me. What should I tell him ? Is their decision Correct ?