Electric69,
The Feds have to prove he "was".....the pilot does not have to prove he "was not".
Now in the UK...perhaps Ireland....the opposite might be true but not within US FAA proceedings.
However, a review of his time card, payroll sheet, job description, evaluations, letter of appointment, all better point out how the piloting was "not" his primary function at the firm. The flying part better be way..way..way...down the list of responsibilities in importance, pay, time, and compensation.
The FAA does have the power to demand records from the US firm owning and operating the aircraft. Add in the requirement for drug testing and the like if applicable to the operation and there might some thorny issues to be discussed.
Knowing the general attitude by EU Tax and Aviation authorities towards non-EU aircraft operations within the EU....this will be an interesting case that will no doubt draw plenty of lightning bolts from the heavens.