is that an instructor carrying out the training flight for revalidation, doesn't have to sign the person's logbook.
If it is a training flight, then under current regulation, to be counted for licence issue or revalidation etc, it must be signed by the instructor giving the instruction:
AMC1 FCL-050 (4) instruction time: a summary of all time logged by an applicant for a licence or rating as flight instruction, instrument flight instruction, instrument ground time, etc., may be logged if certified by the appropriately rated or authorised instructor from whom it was received;
If you refused to sign for a training flight that the candidate had paid for, then the term fraud comes to mind.