I would file with OWCP regardless.
Did your friend ever receive an SF-50 (might be a PS 50 in the Post Office) with a formal job title and job series number? If not, then that light or limited duty job is not a "real" job in the Federal or Postal Service.
I would argue that the WEC award was erroneous because the job in question was not a "real" job in the Federal or Postal service, if it had no proper classification associated with it.
Your mileage may vary. But, I think it's worth a shot, if the shoe fits.