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?
I have used recovery programs before and they work. I am not sure what O/S he is using, but even if he copied over the wallet.dat file in the appdata folder, it can still be recovered. Easy Data Drive Recovery is what I used. I could walk the OP through it on a conference call. Another poster was absolutely right - avoid using that drive - if the file has been deleted than that block is open to be written on.
i am using windows 8, unfortunately. there's no shadow copies and things like that. the other thing is that it took me 1.5 weeks before i discovered something was wrong, that's why most of the data has already been overwritten.
i tried like 10 different recovery programs, they all came up with the same result.
however, i found other fileswuth names like "rev0043.dat". out of those approx. 50 files i was able to recover about 7. i am holding on to these hoping they might be good for something. but the wallet.dat definately seems to be f-up..