I don't have anything against instructors who only hold a PPL.
However, flight training must be legally completed if for no other reason than to provide a good example to the student.
France is not the only place where instruction is not considered to be aerial work or commercial flying. As far as I am aware, JARs say that is the case as does ICAO.
If this PPL was a member of a flying Club or Group that operated on a not-for profit basis and where all the members owned the club and it's assets (if any) then there would be a good argument to let it continue because in efect the only payment would be for the aircraft and all members would be treated equally and all instructors would operate on the same basis etc etc.
However, the organisation that employs this person is set up for the purpose of trading and making money i.e. it is a Company.
No problem with going back to the days when I trained and the most experienced instructors were PPLs.....provided it is legal and above board.
Regards,
DFC