DJI are usually reluctant to try and track owners via serial numbers etc.
Maybe they will come through when they get a chance to look at your inquiry.
We can usually plug the drone into a computer via USB cable and download photos / video, so I guess this may be the case.
You should be able to read files on the micro SD with it plugged in, not sure about mac devices, when you open up a folder like 'my computer' on the mac, does it show the drone listed as a device in the system ?
If so, you might be able to open that, find the folders inside ?
Some owners put a folder on the micro SD with their info, not a lot, but some figure why not.
The mini doesn't require registration with FAA, and in that process afix a registration number / label to the drone, but again many wise pilots at least put a label with their info somewhere on the drone, usually under (or in the minis case) inside the battery compartment.
Not sure how the mini charges, but could well do so like you found, but maintenance things like the DJI Assistant functions are accessed like this too.
Thanks for pursuing this for the owner, hopefully it can have a good ending.
It would hurt to lose such an investment, even the lower cost mini.