You could sue them for something maybe harassment or defamation but based on your brief statement, I am not sure what your damages, if any, might be. If they are violating Facebook's Terms of Service, you can report them to facebook. More likely any standing to file a lawsuit may reside with your father.
This is not legal advice but a general comment on society based on a limited set of hypothetical circumstances. No one should act or refrain from acting based on these comments without seeking appropriately licensed legal or professional advice. The author disclaims all liability in respect to actions taken or not taken based on his comments.
One of the best things about the Internet is that everyone has a voice. One of the worst things about the Internet is that everyone has a voice.
I often counsel clients that there are trolls and idiots on every forum and social media site. My best advice is not to engage in communication with them - it only adds fuel to their fire.
If your father is truly being harassed (and your facts posted don't seem to support a cause of action) he can try to get a restraining order. That could end up costing him thousands and not even solve the problem.
Sorry I don't have better news for you. Deep taking the high road with your posts and leave the trolls alone.
This post is provided for general informational purposes only and is not intended to be legal advice specific to you. This general information is not a substitute for the advice of an attorney in your jurisdiction. The attorney client relationship is not established by this post.