Weird indeed ! The magnet probably is the only explanation since you had checked the tape and the guy tried several times, with other tapes too. But it had to be a strong magnet and in very close distance to completely erase the tape, and still i guess there would be some music / sound leftover somewhere. Don't know. Sounds unlikely to happened but not impossible though.
I don't think i have ever had any tape damaged in post that way or another. From time to time, I have recieved some tapes with extreme unbalanced volume levels between both channels at a side or both, but that's a dubbing problem already existed likely, especially since other tapes in the package were fine.