Different universities have different rules. There's no standardisation the way there is at school. So no point asking us to generalise.
I think maybe the most useful thing to do would be to get her to understand what she's done wrong, so she can at least show her examiners she understands now.
The problem seems to me to be that she (and you) both think the first assignment has some relevance to the second. It doesn't. Forget about it. It has nothing to do with the situation at all.
The issue is that, every single time you take someone else's research or idea and use it, you have to give credit to them. Otherwise, you are passing it off as your own, and that is plagiarism.
If you referenced correctly in one essay, and then plagiarised the same source(s) in another, it's still plagiarism; the fact you'd done it right once doesn't mean it's ok to do it wrong the second time.
Imagine if you'd done a maths question, and you'd slipped up and mistakenly claimed 4+7 was 10. There would be no point in saying 'but look, last year, I didn't make this mistake: I knew it was 11 then!'
It's similar here.