I think my PhD thesis has been plagiarised. What are the professional steps to take? Background: I completed my PhD last Christmas and received it later on in the year.

My supervisor was very absent (there in person, but not at all a mentor)/not very supportive during my PhD. Early on during my PhD I wrote a paper with one of my fields world leading professors, who was a friend of my supervisor. 

My supervisor regularly discussed my work with this individual and suggested that I send my unpublished manuscripts and thesis to them to get feed back. The professor also visited the group, where they read my draft thesis. They did give goodfeed back and I considered this professor a friend and a mentor. 

During last year it came to my attention that one of this professors PhD students has produced a near identical thesis to my own. They do reference my first paper however, and then go on to tackle the same problem using the same method (devised in my paper). At that time, I spoke to my supervisor, who had a conversation with this professor. They claimed that there was no plagiarism and that it was simply a coincidence. My supervisor was happy with that explanation and said it should be dropped. I'm convinced that it is more than that (even the chapter topics are the same), and I have email proof of sending works to this professor. But have not proof that it made it to their PhD student. 

My PhD department was terrible with lots of shady stuff going on (I complained, but it all fell on deaf ears). After landing a job I just decided to put it down as a bad experience and thanked my luckly stars that I was done. 


However I've been recently thinking about this, what is to stop this person of turning this situation around and trying to accuse me of plagiarising them? How do I protect myself? 
Should I bring this up with somebody else at the university? If so, who? Would emails (forwarded on to an auxiliary gmail account) be sufficient
evidence? At no point did this professor state that their student was working on something similar. (nor did I know of this students existence)
Would they wonder why I hadn't brought this up sooner? (its been several months) It annoyed the hell out of me, but I was just glad be done and focus on my future career. I had complained about other stuff going on in the department, but no one was interested and so I figured that this too would be ignored.


I'm so glad to be away from this toxic group, but am now worried that this could be completely turned around and add insult to injury.

Allow me to clarify: I took the method that I devised in my first paper (with which the professor was a co-author) and applied it to a very specific problem. I then wrote this up and sent it to the professor and asked him to review it (as it is/was quite an important contribution to the field and as stated above, over the years they we happy to review my work. I had also hoped that they would be a co-author again, as it extended our first paper in a significant way). I then completed my thesis (said professor read my draft) and the aim was to apply this method to other specific problems, however it turned out not to be possible (as I proved/discussed with said professor) and so this manuscript (and first paper) became the cornerstone of my PhD. I planned on publishing the second manuscript after my PhD submission was out the way. It turns out that this work is also the basis of his PhD students thesis, which was submitted only a few months after my own. They do however reference my first paper (as, obviously, do I) but then go on to do the work in my second manuscript. When you compare the thesis, it is the same chapter lay out as my own: first paper broken into several topics each forming a chapter. Manuscript broken into several topics each forming a chapter. Each of there chapters covers the same topic (in the same order and very similar chapter structure) as my own. This is what I mean by "near identical".
The best course of action would be if you can somehow elevate this to the journal/conference paper level. Was your newer research (the one they have plagiarized) published in a journal or at least at a conference? Is perhaps their (plagiarized) research published in a journal, not only in PhD thesis?

Monitor the student in question and his publications for a while. If he tries to publish that stuff that he lifted from you in a journal, then it is the time to start the action, and simply notify the editor of the journal or a conference about the suspected plagiarism. This way, it will be handled by people not connected to your previous lab and indeed plagiarism in paper is considered to be really bad stuff for one's reputation (often PhD theses are not publicly accessible, papers are).