PhD (Dr.) student willing to finish thesis after several years of inactivity A few days ago one of my former PhD students (in fact Dr. since I'm in Germany) contaced me and asked me, if there is any chance to finish their PhD thesis. The work was done at a different institution where I was active years ago and I was one of the two advisors.

A rough outline of the facts:


At the time the work was done, it was quite innovative. The work was done in an interdisciplinary field and the person had to dig deep into "the other field" and did a good job. There was a tangible result and an evalutation was performed based on a small group of participants.
The tanglible result was novel at that time, the evaluation of the new system was in line with current findings at that time and confirmed the usefulness and practicability. 
The work was not submitted since it was not completed when the contract of that person ended, and after working in the new job, it took a while (~1-2 years) to bring everything to paper.
When this was done (about 3 years ago), the person was in doubt whether "the results are enough" and "whether the validation was proper enough". My feeling was it was enough to at least obtain the grade, even though one could critisize a few things (e.g. size and quality of the study participants), but it was in line with many other theses in this field. The person told me that my remarks would be considered and that I would be contacted if any decision was made. 
Now (about 8-9 years after the work was done) the person contacted me and asked, if I would see any chance to "bring this work to an end" - either with the "Dr." title (no matter which grade) or to finally dump it. The person is willing to add some extra work (e.g. doing another evaluation study), but the system developed is outdated from todays point of view and I would see little benefit in an additional study.
The "Dr." title is quite helpful in the field the person is working in. 
The technical system itself is still quite unique in the field.


I don't want to bias your answers so I would prefer not to tell at the moment how I responded, but I would appreciate thoughts and experience based on the following questions:


Did you ever experience a PhD handing in the thesis many (>5) years after completition? If yes, was it accepted?
If it was accepted, has it been evaluated based on the state of the art when the thesis work was done, or based on the current state of the art?


Bonus question (but opinion based, therefore just as a bonus ;-) ): How would you proceed? (Keep in mind that the was not done in a structured PhD program but as an employee in a research project for a limited time. And we are in Germany.)
Did you ever experience a PhD handing in the thesis many (>5) years
  after completition? If yes, was it accepted?


Our department actually just went through this. Our process involved a justification of why the gap occurred, a defense of the student's standing both at the time the gap occurred and now, and a defense of the scientific merits of the research (and how it wasn't completely stale).


  If it was accepted, has it been evaluated based on the state of the
  art when the thesis work was done, or based on the current state of
  the art?


Current.