We had this on my mates RM 250, it ran fine, then suddenly it would not start.
Unlikely to be the coil or CDI.
Check the resitance of the stator and coil with a multimeter, the range of settings are in the manual.
The distance between the pick-up unit and the flywheel needs to be between .05 to 1 mm (maximum).
Also ensure the earth connections are clean.
I would also get him to completely clean the carb and make sure the pilot screw is also set correctly (normally 1.5 turns out from fully in, but check the manual), easy to do and perhaps the first job as the easiest and cheapest. Then check the spark plug (or replace it) gap and that it has no carbon to allow current leakage to earth.
The RM had a mucky pilot jet and combined with a large gap between the pick up and flywheel. He had bought a new stator, we cleaned the carb and closed the gap for the pick-up to 0.5mm and it fired 5th kick with choke.
Happily riding now but it cannot catch a KX