The external drive he used is a Unidisk model suitable for Apple II series and Macintosh. The difference between the external 800K drive for Macs only is that the Unidisk model has an extra PCB between the cable and the floppy drive. It could be that the FDD inside the enclosure or the PCB is faulty.
One could open the enclosure to inspect it and better try it without the internal PCB: so plug external FDD cable directly in the floppy drive instead of running it through the PCB.
Anyway, like I wrote before, it's probably the Bourns filter or SWIM controller if you can't get the internal drive to work anymore.
Test the Bourns filter for, continuity: it should read around 24 to 25 Ohms exept for the ground pins and both side of the chip: these read zero of course.
The Bourns you need to test is just above the FDD connector on the logic board.
Keep us posted.