So we figured it out. The problem, as Nick Davis pointed out, is that the root folder is only visible when connecting through adb. My colleague does not have a development environment and does not want to install it.
Instead, he installed the Text Edit app on his device, mailed the text to himself, copied it into a new document and saved the file directly to /sdcard/android.sdktester.json
(which is an alias for /mnt/sdcard/android.sdktester.json
).
This was practical only because there wasn't a lot of text involved. For large files, I suppose he could have copied them to a different folder through the USB connection and used a file manager like @ssantos suggested in a comment to move it to the right folder.