Researchers had a paper retracted due to plagiarism and no one at their university is aware of it
If you were in a position of more influence (e.g. a faculty member) or if you were a student at a university where you could be confident that the general attitude coincides with your own (such as most universities in the USA or Europe, at the very least), then I wouldn't hesitate a moment to notify the dean, and escalate the case further as necessary.  No respectable institution would turn a blind eye to this, not least because of the eventual embarrassment when it comes out in the press that they graduated a student whose thesis was known to be plagiarized.

It's hard for me to say what you should do in your situation, because I've never been affiliated with a place that didn't "have a firm attitude about plagiarism".  But given that the indictment of plagiarism is already public, you might approach this from the angle of protecting the institution.  That is, you could cautiously approach the department chair and explain that you're worried about the negative publicity the department (and the university) may be subject to based on what is happening.  You don't need or want to suggest that you're going to approach the press and create that publicity -- everything is already out in the open, anyway!  You also shouldn't suggest what the consequences for the plagiarists should be.  You're only working to protect the good name of your institution.  In this way perhaps you can address the problem with someone in authority such that you are both on the same side.