You write that you are "helping a PhD student to write a portion of her PhD thesis".
Clearly from your question and comments this isn't a stapler thesis: this isn't a case where you and the PhD candidate have co-authored papers, where your work would be in the thesis and explicitly attributed to you.
In your other comments, you make it clear that you have written passages that are in her thesis. That goes way beyond just doing collaborative research. If it's in the student's PhD thesis as her own writing and her own research, then she is falsely representing that she's done the research, and she's falsely representing that the thesis is all her own writing.
In any university that I knew of up til now, that would result in the thesis being failed.**
I think you need to find out the rules of your university pretty quickly, because although the PhD candidate will get the worst of the enforcement, you could get hurt by this. This is now your responsibility to put right, for your own sake. That's because if it is against the rules, then you have unwittingly been complicit with the PhD student in what, upon submission of the thesis, would be the breaking of those rules. I think ignorance of the rules is unlikely to be taken as innocence in any ensuing disciplinary proceedings. At best you could make a case of being led astray by trusted colleagues, but that's going to damage your relationship with them. It is possible, as things stand, to extricate yourself from this situation without too much damage.
So don't go to the candidate or her supervisors to ask for their interpretation of the rules yet (if at all). Find and read the relevant rule yourself. They are almost certainly on your university intranet, if not the outward-facing website; failing that, ask in the university library.
Here, for example, is the rule for PhDs at University College London (UCL):
The work in the thesis submitted by a student must be their own work and the submission of a thesis for examination will be regarded as a declaration of that fact.
The thesis may include collaborative work, but this should be stated as such, and must be written up by the PhD candidate, not the collaborator(s).
If, as I suspect, co-authorship of a PhD thesis at your university is not allowed, then you need to insist to the PhD student and her supervisors that the passages you've written be removed from the thesis. Do it gently, politely, in a collegiate spirit, but do it unambiguously. As you've done genuinely new research, you'll be submitting the words you've written to a journal, so tell them that - that should be enough for them to be sure to remove it from the thesis. Keep the paper trail of all of this (including the emails or other records of when you've sent your material to the PhD candidate in the past), to protect yourself.
** though through the comments below, I've learnt that the University of California in San Diego does allow some co-authoring, providing advance permission from the Dean of Graduate Studies has been obtained.