Its kinda vague - but its worth considering a few things. I'd start (and finish) by saying - focus on the victim not the crime. If you read nothing else, read this.
Moderators have fairly limited access to PII. Lets assume anything you have in your profile, as well as your IP address is visible.
As far as I recall, there's only one real case where Stack Exchange, Inc's been asked to help out law enforcement, and this was probably initiated by them, not Stack Exchange. Jaydles has quite kindly shared what Stack Exchange's policy on these things are - its worth reading this post, but I've copied out the official policy
We take your privacy seriously, and are extremely reluctant to share private information if it can possibly be avoided.
We comply with any legally enforceable requests for information from law enforcement agencies.
This happens very, very rarely. I have more than enough fingers to count the times this has occurred since I started working here a year
and a half ago. I wouldn't need a single toe, and I'm pretty sure I
wouldn't need both hands.
There are many circumstances in which we may be legally prohibited from sharing such requests. The NSA cases have been most widely
publicized. But the more common cases are much narrower, such as those
where a judge has determined that the details of the investigation
would undermine law enforcement's ability to proceed without risk to
innocents, etc. But the most common examples are probably grand jury
subpoenas. These are also the most benign, as grand jury proceedings
are sealed predominantly to protect the accused from having personal
details dragged into public view before the government has
demonstrated reasonable cause to do so.
But lets talk... common sense.
If your friend's a victim, talking about it in context is fine. You could use a sock (it gets a little messy if you want to select a right answer, so a proper throwaway account would be nice, but not necessary). I'm doubtful, except in the possibility of imminent danger anyone would stick their nose in and call the police. They might advise you do though.
If the focus is on helping the victim, and the crime is in context, the end result of what happens to the perpetrator isn't important. "This happened. I don't care about the perpetrator - I'd like to help my friend" would help focus the answers somewhat.
So, chances are no one's going to hunt down who you are, and who your friend, or the perpetrator is, and our ability to do so is likely minimal. If you still need to - there's options.