Parents attending thesis defense?
Open to the public means ... open to the public (presuming they can sit quietly in a room for 2 hours or however long your wife's defense is). 

Moreover, (edit though I thought I was clear), it's very common in the US for family including parents and spouses to attend PhD defenses.

My wife and parents came to my defense (philosophy PhD USA). As did a friend of ours who was a foreigner living in America with no connection to philosophy. A philosophy PhD defense at least at my institution is them drilling you with every objection and reason that they think you're wrong for two solid hours. 

While I assume a nutrition science PhD has a different format, I hope that neither the committee nor the person defending the dissertation has much time to look around and see who came and to think odd thoughts about the professionalism of the candidate based on who came to their defense.