No doubt this'll flush out the usual barrack room lawyers.....
If a PPL holder operated the aeroplane in such a manner as would have hazarded the safety of the aircraft or its occupants had the FI not assumed control, then I would certainly NOT sign the logbook.
For example, if the pilot flew well below approach speed without correction, failed to recover from a stall correctly, failed to fly a successful PFL pattern.........
But fortunately far I haven't yet encountered anyone whose flying was so poor that I wouldn't sign their logbook. Incidentally, a FI cannot sign anything in a licence, that has to be done by an Examiner.