PhD student failing
Have you communicated with the student clearly about the situation? It's easy to softball negative news so they might not realize just how dire their situation is. Sit them down and straightforwardly tell them that their options are ask for help or get sent home soon, like you have in this post.
Currently, since you (and the future examiner) have no insight into the reasons for the low performance, you have no indications that it would get any better soon or that the student would be capable of catching up in time to defend their thesis. Therefore the transfer is working as intended: to catch these cases early so that the student does not waste another two years to no end. It will feel harsh in the moment, but it is not in the student's best interest to stretch out their failure for another two years (I imagine they don't feel great about their work either).
If they can explain what went wrong, establish a plan for stopping it from happening in the future, and demonstrate improvement by the time of the deadline, only then would I consider pleading for leniency with the examiner or administration. Essentially, inspire yourself from standard management techniques and put the student on a performance improvement plan.
The previous answers are excellent, and so I'd like to only add that it is worthwhile putting things in text. In case you have an in-person "heart to heart" in which you lay out the options and have a discussion with the student, sit down later on and summarize the discussion in an email that you send to the student. If you want to do it the other way around, that's fine as well -- sometimes a student needs to see written down and have time to digest something before they understand the gravity of the situation.
I suggest this for two reasons:
For some students, especially if English is not their first language and/or if they come from cultures where the culture is so that they say "yes" and nod to everything a professor says, having things written down provides a different level of reality than just hear someone say something. It also gives them a way to go over it again a couple of times to really understand what you are saying, if necessary with a dictionary.
For you, it gives you something you can point at if push eventually does come to shove. You can now reference an email or memo that can be looked at and that has a concrete date, in case you need to write the student about not performing the tasks you had laid out previously, or if you are asked by administration later on if you can back up your claim that the student has consistently underperformed.
I've been in your situation before, and at the time put the student on academic probation for two semesters after having struggled with the student for a year or more. That got her attention, and in the end she wrote an excellent thesis, independently and on a topic of her own choice. I admit that that was one of the worst things I had to do as a PhD adviser -- our students are our kids too, in some sense. But at the same time, dragging them along is also not a solution: they need to pull themselves out by their bootstrap, because (i) we can't just give them a degree, and (ii) there is a better student somewhere who could be admitted if we let students who don't do well go.
AFAICS no-one has mentioned clinical depression (can be triggered by lack of sleep or of course other reasons). The unexplained disappearances make me wonder. Is your student properly hooked into a medical system? I am not an academic, but when I was a student the Uni had a duty of care in this regard. This might justify an intervention even though they are adults.