Account hacked but not mine....

So my brother had to stop playing WoW a couple of months ago. He canceled his account and all. This past Sunday I saw his characters online, each one logging in then out. I wasnt sure if it was him so I sent the character a message and asked if he was back playing again or was this an account thief. Of course I got no response. So I messaged back, " Hi account thief, how are you today?"
I then call my brother to confirm that is wasnt him and of course he had no idea. So my problem is this. I know there is an account out there that is compromised but it isnt mine. So what do I do? Do I send an ingame ticket? Do I do nothing? I know its not my account and my brother could just call Blizzard to get things straight but hes not able to play right now so his motivation to do it is lacking. It just bothers me to know that his account was stolen and theres likely nothing I can do about it.
I do get a kick out of sending the account thief random messages knowing they wont respond.