OK here is my intrepretation;
I've just checked the FAA regulation 61.75 which deals with piggy-back certificates
(e) Operating privileges and limitations. A person who receives a U.S. private pilot certificate that has been issued under the provisions of this section:
(1) May act as pilot in command of a civil aircraft of the United States in accordance with the pilot privileges authorized by this part and the limitations placed on that U.S. pilot certificate;
(2) Is limited to the privileges placed on the certificate by the Administrator;
(3) Is subject to the limitations and restrictions on the person's U.S. certificate and foreign pilot license when exercising the privileges of that U.S. pilot certificate in an aircraft of U.S. registry operating within or outside the United States; and
Tail wheel is not a rating, it is a training requirement in accordance with 61.31(i) which requires a logbook endorsement.
So it is not a privilige which is placed on the person's certificate.
If the person involved in the accident had a logbook endorsement from a FAA CFI in accordance with 61.31 (i) they should be in the clear.
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