I run an Iowa Hawkeyes forum and one of our users has continually posted "premium information" from private Iowa forums on a national recruiting site. We've been contacted by the individual that runs that particular site/forum asking us for this individuals email address so he may ban him from their site. While I know he is breaking copyright rules and potentially opening us up to be sued (although we comply every time they let us know about it), should we protect the user and not comply with the email address request? What would you do?
What does your privacy policy say? I would try to stick to the spirit of that. This is the minimum that I would do: Ban that user for posting copyrighted content. Remove all questionable posts. Tell the other operator that that is what you have done and that your privacy policy prevents you from disclosing their email address, short of a request by the courts. If your privacy policy is more open ended then share it. My guess is that they are not going to sue this user. They probably just want to ban him from their service for violating their ToS.
Did he posted that content also in your forum? If so then you're responsible for deleting that content. Regarding giving out the email the one who runs the other forum should check the ip address of that member. Timo