That is exactly how I have it set in the prefrences. So, are saying that the dng file will not preview before importing? That means she has to import all and then delete which I was trying to avoid. Or leave as is and use the jpeg preview there to import the correct dng file which seems a bit odd.
So it seems that you are referring to previews in the import grid. If there is no preview there I can only think of the case where no preview JPG is embedded in the DNG file. I don't know what the Leica does with the DNG files.