This is not your ax to grind. The parent or legal guardian of a 15 year old is the only one with standing to raise legal rights on behalf of the minor. Defamation requires proof that incorrect statements of fact, not opinion, were made to third parties and that it caused harm to you. Here is a link to a guide on defamation law to help you determine whether the law applies to your circumstances.
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This answer is offered for informational purposes only. It is not offered as, and does not constitute, legal advice. Laws vary widely from state to state. You should rely only on the advice given to you during a personal consultation by a local attorney who is thoroughly familiar with state laws and the area of practice in which your concern lies.
I would recommend that your friend remove the 20 year old as a "friend" from Facebook. Also, if the comments are false and malicious, which it sounds like they are, your friend may have a cause of action for libel/slander. If legal action is to be taken, the 15 year old's parents will have to pursue the cause of action on behalf of the minor.
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