Threatening is a line best avoided. Embarrassment however is a wonderful thing. Peaceful protests with placards and leaflets would possibly embarrass his parents in their street without doing anything illegal. As long as only the truth is told there is no risk of libel, as long as it is done quietly and peacefully there should be no risk of any laws being broken. Perhaps a letter to the local newspaper about the affects of social media on fraudulent activities might generate more response.