While it is a sad and unfortunate happening, there is nothing that can be done. The characters would have been deleted by "someone". If it was left on, could there be anyone else using that console and deleted them, whether on purpose or accident? It also could be that the XBL account was hacked ([i]not uncommon[/i]), so the first thing you should be doing is changing the XBL account password and potentially setting up two factor security with them. Has he shut down the console completely and then started it back up again to make sure they were deleted? I saw one person this week think his were gone and then when he shut everything down and restarted they were there. Bungie does not replace any lost or deleted characters, items, etc. It is a well documented policy, and one many game developers adopt.

Unfortunately, Bungie doesn't offer any options to recover deleted characters. Account security is solely between you and your game service provider, in this case, Microsoft. I know this isn't the news you wanted to hear, but this is the same thing you will be told by Bungie. It may be no consolation, but, I -think- the game still remembers the exotics you've had, so you can recover those (at a low light level) from your kiosks along with any shaders/sparrows/emblems you had.