There is no requirement for an examiner to check that the pilot has a valid medical when the pilot renews a SEP class rating by experience. There is no requirement for the pilot to hold a valid medical when completing the administrative act of getting an examiner to sign his/her licence. What they can not do is exercise the privileges until they do have a medical.
The examiner is required to;
1. Check that the existing rating is within it's period of validity
2. Check the logbook for the required experience and if applicable the flight with an instructor.
3. Sign the licence page
4. Fill in the requried form and send it off to Gatwick
I and most examiners I know will do a quick check of medical, licence expiry (JAR ones) and such like so that the licence holder can be assisted in avoiding something going out of date. However, there is no requirement other than what I indicated above.
In my opinion, the pilot should simply be advised that you can not sign the revalidation page this time because of the fact that the paperwork is not correct. They should contact the CAA for advice and either get the SSEA on the NPPL or a Class 2 medical. The CAA say that in such a case, they will sign the rating revalidation for this pilot once "the paperwork problem" has been sorted out.
Unless the previous examiner signed the revalidation outside the validity of the previous rating they did nothing wrong.
Regards,
DFC