NOTE: This is not legal advice - that would depend on various laws in various jurisdictions and is best left to lawyers.

Personally I would politely explain that you cannot disclose a user's personal data without a court order. If they go through that step, they may actually have a right to expect you to provide it. As it stands, I would not hand it to anyone based on an unsubstantiated claim. Just my opinion.

As mentioned above,
This is not legal advice - that would depend on various laws in various jurisdictions and is best left to lawyers.

Disclosing a user's personal information without a court order could make you legally liable towards the user who's data you disclosed.

Hello,

First of all, you shouldn't have let someone post defamatory content, ... I know it's easy to say, and I am not blaming you. At my sites, freedom of speech comes second, after "my" right of not getting into troubles, because of others.

That being said, I don't know in which country you are, but there is certainly a watchdog / data protection official organism. Write to them, expose them the situation, ask them their advise. Then, you can use their answer, to protect yourself.

One more thing under the IANAL head: resist the temptation to hastily delete the user's information so you cannot comply with the court order. This is A Bad Idea. (I think it was LegalEagle that recently had a discussion of this topic, prompted by a user whose boss had asked them to do this very thing.)

I echo what's been said.
You should take legal advice for your region.

Amazing advice - Thank you all so much.

Under the ordinary (most countries have data privacy laws) you are REQUIRED to hold and protect any PII (personally identifiable information) and NOT REVEAL IT.

Not legal advice, it's just the law!

You should only respond to a court order from your jurisdiction. If you get one then GET AN ATTORNEY and let them deal with it.

Same disclaimer, NOT LEGAL ADVICE.

It's off-topic, (but might help).

Some years ago, I found the site of a law school (I think), in the US, and from what I understood, they were offering free lawyers to help you in any kind of cases. This was free , because they were students , so "may be" , not as good as professional lawyers, and "may be" not experienced enough to deal with complex cases. I guess that the catch, is that you are accepting to be assisted/represented, by a law school student, and its consequences, and in the other hand, it was allowing students to gain experience, with real cases and situations.

I also remember, that a donation was welcome, but not mandatory.

However, I forget to bookmark this site, and I can't find it anymore. Does it sound familiar to someone? I thought it could be great for small businesses, who can't afford "full" lawyers.

Now, it's possible I totally misunderstood what this site was offering :-/

Just to add to the complexity of all of this, a court's request, might be illegal :)

If I don't make mistake Switzerland, was recently condemned because a judge ordered a journalist to reveal her sources, which she did. Then later, The European Court of Human Rights (ECHR) ruled against Switzerland for doing this.