It's certainly the right thing to do. You can call the police and ask them to check on the person to make sure that they're all right. Don't be surpised if the person who is abusing your neighbor lies about it or the neighbor being abused is afraid to say anything. Although there could be other incidents with this person.
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You are not under any duty to make such a report, so it is really a philosophical question as opposed to a legal one. Notwithstanding, if you make a report and some legal action should arise as a consequence, to wit criminal charges, you should expect to be subpoenaed to testify as a witness.
Another issue that you must contemplate is the fact that many alleged victims of domestic violence are either in denial of the abuse, or will deny the abuse to prevent it from becoming pubic, breaking up the family, and the like. With that said, a good-intentioned report by you may compel the alleged perpetrator and victim to seek reprisal, legal or otherwise.