That would be vandalism. Your friend's rights stopped when he inflicted damage on the other system's hard disk. Had he stopped at logging the MAC address and barring further communication, he would not have had a problem. What a lovely tort the victim would have if his business records were destroyed. There might even be criminal penalties if the act occurred in a state with the appropriate laws on the books.
Yes, I know. I heard the story through the grapevine. As I understand, the network in question was also heavily secured against unauthorized access, so I don't think anyone's hard drive was wiped.
But yes, under Virginia law, it would be a crime. We have both laws for computer network trespass and property destruction here.