Plagiarism in undergraduate thesis
First, the use of the earlier material may have been plagiarism or not. If the methodology was &quot;common knowledge&quot; then it need not be cited.
Second, if sections were copied, verbatim, without citation, then it is still an error but one of copyright violation, not plagiarism. Copyright is about words (actually &quot;creative expression&quot;). Plagiarism is about ideas. Not the same thing. But both need to be avoided.
But, you don't really need to take any action unless you are somehow responsible for academic integrity at the institution. Even then, it may be too late to impose any penalty. Vigilanteism isn't called for. The responsibility for evaluating the thesis was with the university at the time and while they may have failed, it isn't your responsibility to raise a now-dead issue.
If you need to give her advice, talk about the dangers to her career from repeating such things in the future. But I think you can, with a clear conscience, let the past be the past. And be sure that you understand and communicate the difference between plagiarism (mis attribution of ideas, even if paraphrased) and copyright infringement.