Not an awful lot you can do. If you can get an email sent to a pc then you might be able to read the 'header' information, which will hopefully contain the originating email address, but it might not, and unless you can get hold of the email, that won't help you.
I know on land lines that even when a number is 'withheld' it's still transmitted but with an extra code that stops it being displayed, with the right gear you can still read it. I'm not sure if that is true of mobile phoes though, and even if it is, you'd need some pretty specialist gear to be able to read that information.
Your GF needs to make the complaint, nothing else is going to work.