What to do (years later) with otherwise good student who has accidentally plagiarised part of PhD thesis?

Here's a not so hypothetical situation. International student x is very talented but comes from a background where technical writing is not taught or understood very well. She writes a great thesis with a good literature review and nice results. However, the results are based on two key papers from previous students in the group. She decides to give credit to the papers in a special chapter, which she starts by saying "I need to give credit to this and that paper" and proceeds with copying paragraphs wholesale to describe what those other students did.

This was a few years back; X is now faculty at a good school and she contacts me (past advisor) in teary-terrified voice to let me know that she plagiarized in her thesis. I am now in a panic as well. How could I miss those? And how could she do that?? We both risk losing our jobs, and she is at risk of losing her degree as well (which, by the way, was a very strong thesis with a good number of top journal publications).

As far as I know there's no process for revising a thesis after it's been submitted and I don't know what else to do short of turning ourselves in - which I feel morally obligated to do.

Please advise.

Edit Thanks all for weighing in on this. I spent the night going through the thesis and there appear to be three more sources that are suspect of being plagiarized, all in the same wretched chapter; one is a thesis of a colleague, the other is a textbook and the third is a book I wrote a while back. So this is more serious than I thought.

She has unfortunately not used quotes for the material, i.e., instead of saying "[paper i] says ," she just went on with "[paper i] says this and that."

She has not been accused of plagiarism by anybody. I am guessing that she has finally come to grips with good writing standards and upon looking through her thesis she realized that her "summary" was actually plagiarism. I have every reason to believe that she did what she did in good faith (she has proven her honesty on many occasions).