Screen shots of the comments before the user had time to delete them. We've sacked people at work but I imagine it's an HR/employment law minefield to actually go through with it.
To my knowledge they're not allowed to hand over information as I guess this would have been a civil case, not a criminal case - not to mention they would require warrants, reasonable cause and a rather large cost involved.
After all, you could say that someone else was on your account without your knowledge, it's up to your employer to prove your guilt.
In the past I've been 'fraped' with comments so vile that I could have lost my job over it. I didn't, but where do you draw the line of work and social?