As the legal requirement for revalidation by experience is 12 hrs... blah... and a 1hr flight with an instructor, arguably, if both you and the 'student' are still alive at the end of the 1 hr flight, the requirement has been met and thus you should sign all the necessary bits of paper!
"should" is incorrect - it implies it must be done. It is completely up to the authorised person (Instructor/Examiner) whether they choose to or not.
If you are appointed by the CAA to sign certificates of experience and the candidate has met the requirements and you refuse to sign, the candidate has the right to appeal under CAA Regulation 6.
Appeal against what exactly ??
The candidate is perfectly able to approach other individuals (Examiners), or the CAA themselves, to get their Revalidation paperwork signed.
They could, possibly, put in a complaint to the CAA about the (lack of) action on the part of the Instructor but I seriously doubt it would be upheld - and may even lead to consequences for the candidate themselves.