The pilot displayed adequate skill during the flight you observed only. What happened thereafter was entirely the pilot's responsibility.
If you don't like the standard of the flight, tell the pilot that you won't sign his logbook. If there are any doubts, then say you will sign 'UNSATISFACTORY' in his logbook - or not sign it at all. His choice.....
But who are these bleeding heart barrack room lawyers who are doing all this whingeing? I haven't heard of any cases of pilots not performing satisfactorily during the continuation training flight, have you?
NB: Any reference to the male gender in the above post should be taken to apply equally to those of the female, ambiguous or uncertain genders.