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Was her passport stamped by the IO at  the port of entry? If yes, just provide the copy of the entry stamp.  Then, write something like " No I-94 issued-Canadian citizen" in the Form I-485 where asked for I-94 information.

If her passport has no entry stamp, I suggest her to go to the local CBP Deferred Inspection Office to get the I-94 or proof. Find the local CBP Deferred Inspection Office near you at this link:
https://www.cbp.gov/sites/default/files/assets/documents/2017-Jun/deferred_inspection_sites_060817.pdf