I cannot address the legalities but I will say that it is suspect.

There was recently a thread where we talked about being able to access the internet anonymously because of the tracking done by Facebook, Amazon, and other sites/retailers.

I had a friend who was stalked online for years by an ex and it drove her off the Internet and then basically underground. While this request could be innocent (looking for old friend, etc) it might be helpful if this person making the request might explain the purpose, etc. Then it would be up to those consider searching if they believe the person on the why's and if they want to help.

Just like most things, this hold be good and it could be bad. Whether it is legal may come down to the ultimate purpose the search is undertaken and what the person does if the username is identified.