Umm, another possibility is that he learned Castilian, and the people in your office grew up speaking some Latin American version. Just the other day a native speaker from Spain told me of his being completely flummoxed by the border guards trying to casually chat him up on his entry to the U.S.
We had Latin American folks in the office too. They never had a problem speaking with each other. Maybe a border guard speaking broken Spanish confused someone, but this woman was just trying to ask basic Spanish 101 type questions.
The follow-up to the story is that I came across the same candidate's resume 6 months later. He had changed the "fluent in Spanish" to something like "basic Spanish".
No job, but life lesson learned.
Someone fluent in conversational Spanish would still be able to speak the basics with a sympathetic, if skeptical interviewer.