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I am sorry to hear about this situation. Unfortunately, claims of inappropriate behavior in custody battles are very common. No doubt, the mother of the daughter is attempting to claim that the father's home is not suitable because it poses a danger from your son.
There is no 100% way to protect him from such accusations. CPS has to look into every claim, so likely he will be interviewed. It is recommended to use an attorney to help him with the interview (the interview would be voluntary). The mother can also be threaten with a civil suit for defamation if she continues to make such false claims. Under New York law, the elements of a defamation claim are: a false statement; published to a third party without privilege or authorization; with fault amounting to at least negligence; that caused special harm or defamation per se. Dillon v. City of New York, 261 A.D.2d 34, 38 (N.Y.A.D. 1 Dept. 1999). She can even be sued now, once the claim is dropped. A threat to sue often has the individual back off, understanding that such claims will have repercussions if made.
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