Backing up another user’s mid-2013 Macbook Air
As the end result is to be able to access any forgotten files from a Windows computer in future, rather than to be able to restore to a new Mac, I would forget trying to backup the drive in its current form. 

The OS &amp; apps themselves would be of no use; &amp; could easily be re-acquired if ever needed, as they will belong to that user's Apple ID. It would also be reasonably safe to assume that any 'hidden' files wouldn't be portable to Windows - unless the machine was previously used for programming/coding. [Let us know if that's the case.]

That leaves you with, essentially, "Documents".
These would to a large extent be cross-platform, in &amp; of themselves - txt, rtf, jpg, mp3, etc etc
I'd say in that case that simply copying the entire User folder [Macintosh HD/Users/username/] would be more than enough. If you copy it to a drive formatted as ExFAT, it can live there as just another folder, accessible from any OS. You will probably be saving 'too much' data that way, but it will save having to make individual decisions on the contents of each sub-folder.

This solution does, of course, ignore any security considerations. The files will be in plain sight on the new drive.