She shouldn't have any trouble entering Canada as a visitor (stamp is good for six months). With an outland application filed from the US, she's as free to enter and exit Canada as any other US citizen. There's never a guarantee, but most Americans don't encounter any difficulty at the border. I'd make sure to use the language that she's "visiting" you, not residing with you. We do know of another couple like us (him Canadian military, her a US citizen) who attempted to enter Canada together, and the wife was denied entry at the border. The agent (who was probably either a jerk or having a bad day) said he knew what she was doing, that she was illegally moving to Canada as a visitor.
I had no issues at the border when we drove in, but it does happen.