Guns are not allowed inside the building. If attempting to bring one in is subject to criminal penalties, that’s fine. Severity of the charge and punishment is probably dependent on whether there’s evidence of intent to cause harm. If it was an honest mistake, he’ll probably get a slap on the wrist.
I hope most responsible gun owners would agree that whether you have or do not have a gun in your bag is not the sort of thing you should ever simply forget.