It could be Steam Family Share… that thing is kind of screwing people over.
All he has to do is make a new Steam account and share his games with the new one. He gets a new steamID and a free copy of the game.
I don’t know if that’s what’s happening… And I don’t know that much about the family share, but from what I understand, this is possible.
[editline]17th March 2014[/editline]
Although… this is interesting:
(from the Steam Family Share FAQ)
Will I be punished for any cheating or fraud conducted by other users while playing my games?
Your Family Library Sharing privileges may be revoked and your account may also be VAC banned if your library is used by others to conduct cheating or fraud. Additionally, VAC-banned games cannot be shared. We recommend you only authorize familiar computers you know to be secure. And as always, never give your password to anyone.
EDIT: I read this wrong before, but still might be considered cheating or fraud of some kind.
So maybe you could report him on Steam?