I suggest a system restore, maybe a patch has gone wrong on his account?
OR he could possibly be the victim of a bug in the game that hasn't been patched yet, therefore when he loads the game, it tries to load a corrupted or bugged file and doesn't allow the game to start?
I'm not sure, but BEFORE trying the system restore, try deleting the patch data by going to;
Game - Game Data Utility - Assassins Creed Brotherhood - Delete.
Then chuck the game in, it will tell you that you need to download a patch and then you're set.
But if that doesn't work, back up all his files on a Flash Drive (USB), system restore, copy the files back to the PS3 and then try.