I've been finding co-pilot really useful but I'll be pausing it for now, and I'm glad I have only been using it on personal projects and not anything for work. This crosses the line in my head from legal ambiguity to legal "yeah that's gonna have to stop".
I suspect he has a different problem which (thanks to Microsoft) is now a problem he has to care about: his code probably shows up in one or more repos copy-pasted with improper LGPL attribution. There'd be no way for Copilot to know that had happened, and it would have mixed in the code.
(As a side note: understanding why an ML engine outputs a particular result is still an open area of research AFAIK.)
All lines are associated to a commit, which has author/commit date. A reasonable guess as to which snippet was made first can be done