There are two things you got wrong with your analysis that are unrelated to the choice between "will" and "be going to":
1. The person is stating what Jennifer said
When you state what someone else says, you are stating what they actually said, not what you believe they should have said. Even if Jennifer hadn't decided, if she said she did, then it'd be wrong to paraphrase
Jennifer: "I have decided"
Jennifer said she hadn't really decided yet.
(Unless you're using the word "say" in a metaphorical sense, to mean something different from what it usually means.)
2. Jennifer might have actually decided
Have you ever heard of a "split-second decision"? People can decide quickly, without much deliberation. And deciding does not mean that you can't change your mind later.
You might wonder, how could Jennifer have decided, if the writer states that "she doesn't know what she's going to do"? The writer might be wrong. But even if they're not wrong, deciding is not the same as knowing the future, unless you are omnipotent. Consider the following sentences:
I made my decision, but I don't know if it was the right one.
I decided, but I might change my mind.
Don't mix grammar and semantics
Sometimes people can fall into a trap of trying to match rules of grammar (or rules of thumb) too closely to objective truths of the world being referred to. That's not how language works — language is for expression. The correct usage of language follows what is to be expressed.