Social Security Questions? Ask a Social Security Expert
Without knowing too much about the specifics of the case, I would suggest that you read the US-Canada Social Security treaty, in the attached pdf file or at http://goo.gl/rKG4Hw. Any reduction in benefits would almost certainly be based upon this agreement and your should understand it well and any arguments you make to the ALJ should be based upon its provisions in addition to Social Security law as well.
I hope this helps and let me know if I can clarify anything or answer any additional questions. Thanks, Jonathan