Well, we figured out what happened:

He erased/reinstalled Bitcoin (alongside other programs) on his computer, because he thought that the wallet/coins are safe on the USB stick (only the client is on the stick, not the blockchain or wallet data).
The wallet.dat is from a recovery program and couldn't be fully recovered.
There is still a (small :/) chance that the private key is intact in there.
Or is there maybe another backup option untried? some autobackup from windows maybe?