I would like to expand on 102008's answer for what myself and my wife do, as we are in a similar situation. Make sure that it appears obvious that your wife is (a) very patriotic about her nation, and/or (b) doesn't quite like the US. Counter-intuitive as it seems, the border guards are not looking for people who want to cause trouble, they are looking for people who want to stay.
Have her wear a shirt with her nation's flag, and maybe she should mention something about American rudeness or some such disdain such that the border guard will just overhear it. She should appear as if she is being forced to accompany you, not that it might be something she enjoys.
Enjoy the trip!
Edit: Considering that in the last decade Americans are not at rude and vain as they traditionally were perceived to be, perhaps directing the wife's comment not at the people but at the government would be more appropriate. Consider government surveillance, militarization of police, etc. for easy targets. The point is not to be rude, but rather to be naively appalled by some aspect of America that would indicate that the wife has no desire to stay.