Summary of facts and other information regarding the legality of the flight:
The Pilot
Fact: a private pilot, his licence did not allow him to pilot a revenue earning flight, whether or not he was himself paid. Even having his hotel room paid for him was not allowed under the FAA rules for private pilots.
Information: according to a post on this forum his EASA medical contained a restriction to day flying only due to colour vision problem, and by extension the same day restriction applied to his FAA piggyback licence because "all restrictions on the foreign licence apply".
The Aircraft
Fact: as a single engine piston aircraft it could not be legally operated commercially at night. A bona fide private flight at night (with a pilot who was allowed to fly at night), would have been legit, but not a commercial flight.
Information: according to leaked messages from the passenger it was always going to be a return night flight.
Jurisdiction
Fact: The aircraft was USA registered but UK based. If flying outside the UK, the pilot had to have an FAA licence and operate to the more restrictive of FAA rules and/or the rules of the country in which the aircraft was flying.
The Type of Operation
Facts: if commercial, the operator would need either an Air Operator Certificate from the UK CAA or a Part 129 Certificate from the FAA. According to the AAIB interim report this aircraft did not have permission for commercial operation form either the CAA or FAA. Nor would any of that permit the aircraft to be flown commercially at night anyway.
Information: it has not been established officially whether the flight was private or commercial. However, statements from Willie McKay as well as leaked messages from Sala, indicate that the flight was de facto commercial.
From the above it follows that: the only potential legal scenario is if the flight was a bona fide private flight (contradicted by the statements of Willie Mckay), the report on this forum of the pilot's medical restriction to day flight is wrong, and the pilot held at least an EASA night rating (unkown). More likely, the flight was illegal for some/all of the following reasons:
The pilot was NOT allowed to fly at night (based on reports of medical restriciton).
The pilot was NOT permitted to operate commercially (fact according to the regulations and airman database).
The flight was NOT private therefore it was an illegal charter (based on reports of Willie MaKay).
The aircraft was NOT permitted to operate commercially at night (fact according to the regulations).
The aircraft was NOT permitted to operate commercially by day anyway (fact according to the Interim Report due absence of requisite certificate).