As far as I can ascertain there is no JAA or CAA definition of "supervision". But if stood up in court following an accident on a solo flight any instructor would have to have shown reasonable care in the authorisation and supervision of the flight. I would not consider authorising someone to fly from a remote strip as due care and attention.
However, all PPL training must be undertaken at a licenced aerodrome, which your friends farm strip is not.
Therefore he could not operate on a training flight (dual or solo) at all from his own strip, he must get somebody else to fly his a/c to a licenced aerodrome if he wishes to use it for training.
Barney
