Well, that may be due to different things. One thing is to speak spanish colloquially, another thing is to write it, and yet another thing is to know exactly the type of answers that a certain language exam demands (one would think that grammatical correctness is all it takes, but there's certain formulas that written exams require).
And yes, Spanish can vary greatly, even more than English. The argentine brand of spanish, for instance, uses "voseo", which means the changing of one of the pronouns and a whole another way of conjugating verbs, not to mention a huge italian influence in its vocabulary (specially in Buenos Aires).
I hope your grandson's girlfriend has a little bit more luck next time! If she reads the GCSE books to know the type of excercise they'll take before hand, she should be fine!
