Thanks to All for the replies.
The user sounds quite computer-literate, likely more than I am, and I'm working on the basis that he's sincere and hasn't damaged it. I sent a link to a Youtube vid of enabling Windows 10 to run Nikon Scan, and he replied he'd already tried that with the same symptom. No error message has been mentioned, and the concept of filling up whatever storage location he's using, possibly with high re scans, occurred to me also. If it was a hardware issue, surely someone else would have experienced it.
I get the feeling I'm going to end up getting it back, hooking it up and finding it runs fine......
Thanks again