I wonder if it'd worth to send her HTML email pulling an image from a server under your control, then look at the server logs for what IP accessed the image.
However, once you have that IP... without a court order any responsible ISP won't give you an account name or address.
Another way, a bit less ethical but nevertheless... if you know of forum(s) where she hangs out and can socially engineer the forum admin to get the IP # off her posts, or send her an email posing as a friend trying to engage her in some random conversation on a forum where you have allies who can get the IP # for you. Again, underhanded, less ethical, but...