What legal action can be taken against family threatens?

The boy and girl agreed to marry after their first meeting. One function (Godh Bharai) was held in the girl’s village/small city. After the function, the boy returned to Australia. However, the boy and girl started having conflicts and found themselves incompatible. Now, the boy’s family wants to call off the wedding, but the girl’s family is threatening to file a case alleging that the boy is a fraud or already married to someone in Australia. All of the boy’s documents, including visa and passport, are valid and confirm he is not married to anyone else. Please suggest what action the boy’s family should take in this situation.