At least while clothed, Centauri and human female look almost eactly alike. The Centauri wears a wig, the human shaves her head. A close examination might pick out differences in the teeth, but the Centauri females are much more similar than the males.
The biggest problem is going to be more social. Gestures and expressions will be different, and of course they speak different languages and will have to cover up any accent. Spies in WWII were often tripped up because of their lack of knowledge of everyday things for the enemy. German spies would get caught because they didn't know who was pitching for the Yankees that season, and American spies would be caught in Europe because they kept switching utensils between their hands or turning their plate while eating. In this case, that will be an even bigger problem.
So, physically, I think your NPC will be perfectly safe unless she must undergo any medical treatment or the person she's interacting with is well versed in anatomy (Dr. Franklin would probably pick her out pretty quickly). She may also be in trouble if she has to share a bathroom or something like that. Her big problem is going to be mimicking human behavior. Here she will have no better chance than any other non-human character.