He had in fact charged the battery the wrong way round so that the polarity was reversed so that when he reconnected it "Correctly" it was actually the wrong way round!

I am actually quite impressed that he was able to do this. In due course I will look into the electrochemistry that made this possible

Fortunately being an old car, the only real damage was to the radio which now no longer works. (I have suggested that we check the back of the radio for a mini fuse but do not have the gizmos to remove it for now)

As an aside, just How well protected against reverse polarity errors are more modern vehicles?? (If at all!) ISTR a spate of reverse polarity type faults some years ago as a result of the bizarre continental practice of designating the live battery cable as being Black! but haven't seen to much of it more recently.