My point is that if she was to identify the external situation as the (main, etc.,) cause of her nervousness, her stated reason should refer to the elevator situation

Ah, okay. I thought you were actually defending the fundamental attribution error, when it turned out you were just critiquing her communication. Or at least, that's what I hope. You still seem to be focusing on "the cause" a bit oddly - what are you going to use the cause for?

Speaking of which :)

If in general it's impossible to decompose the cause of something into external or internal, then the Fundamental Attributation Fallacy is not a fallacy

Well, when it's right, it's not a fallacy. For example, if someone eats my jello mold and thinks "man, he must know a good recipe," that's successfully identifying an internal cause, which nobody could have any quibble with.

The trouble comes in two ways: the first is when people just do straight-up classification into types. "He's angry because he's a member of the type 'angry people.'" is not a good guess, because people are more complicated than that, but somehow (cough brains are lazy) this guess comes up a lot.

The second kind of trouble is when people think they see an internal cause, and then proceed to ignore external causes. From the wikipedia article: "Subjects read pro- and anti-Fidel Castro essays [...] when the subjects were told that the writer's positions were determined by a coin toss, they still rated writers who spoke in favor of Castro as having, on average, a more positive attitude towards Castro." Weird, right? These people just ignored information.

In the anecdote at hand, it's totally fine to say "she was nervous? I bet she had a bad experience in the past." I mean, one has to keep in mind that it's less certain than the recipe thing, but that's fine. A "fundamental attribution error" type guess would be "she was nervous? She must be of the type 'nervous people'," which is still a well-formed guess but is made by the human brain more often than it should be. And then the fundamental attribution error par excellence would be to say "she was nervous because she's a member of type 'nervous people,' the situation didn't cause it."

You seem to be confused about "the situation causing her nervousness" and how that relates to the mentioning of her previous experience at a party. I really don't see how

But her reason actually refers to a totally different situation!

seems like strong evidence for the "cause" (I agree with what buybuydandavis says above about the use of the word in this situation) being the woman in question.

It seems obvious to me that she has beliefs based on her experience at the party that make the elevator situation worrying. This is not a difference in type from, say, one's beliefs about what a gun looks like informing one's nervousness in a situation in which one sees a gun, and I think it would be pretty silly to say that one's external situation was not the "cause" of one's nervousness in that case.