I think my PhD thesis has been plagiarised. What are the professional steps to take?
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).