It shouldn't present any problem. She will have to bring with her, her valid passport, her expired GC, the Form I-797 (NOA1) Receipt/Extension letter, and a copy of her marriage certificate (if her last name is different on her GC and passport).
Those are the documents my wife took with her when we had started the ROC process and traveled a couple of times to Vancouver, BC, Canada. We had no problems either going to Canada, or back to the US.
I would suggest that you make several copies of the Form I-797. We kept the original copy in a plastic page presentation sleeve.
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