Whoever hacked his account committed a criminal act by doing so. Admins don't have to disclose whatever info they find, but it should at least be looked into as they could have been stupid and left breadcrumbs leading them to other users. At the very least he needs his account back so they're going to have to look into fairly deeply to get it back to him.
You are correct in saying that whoever allegedly hacked BitcoinGirl.Club broke the law. Howeer given the lack of apparent financial damages, and the appearance this is an isolated incident, law enforcement is likely to do little more than take a report.
If this becomes a more common occurrence, I am sure the admins will be able to use any information available from this hack.
I disagree, besides it would be more of a reputation issue to me and it would be nice to know who to not trust. What if there's clear evidence linking them to an existing user here and they're allowed to go on free with an unblemished rep.
It would be nice to know who not to trust. You are free to petition the admins to get involved, but I doubt they will for something this petty.
If the admins say they don't know who was behind the hack, whoever is behind the hack would know they are in the clear (somewhat). If they are outed as the hacker, they might destroy evidence for when the police come knocking on their door.