could even be the starter going to earth , really needs to do some basic checks when he returns from his run out , battery may be ok is the dynamo charging is the wiring from the dynamo ok to be honest not hard to find the problem . bit of detective work required paul
used to happen on old engines had a peugeot gti . silicone held it in place until sold in auction . best way to find out give the engine quick clean with brake cleaner give it a run have a look. think you are probably correct not pushed in properly paul
not actually saying it is the dynamo , but a few basic tests could save you wasting money , may just be the terminals corroded on the control box or inside , have had it happen on my vitesse bit cleaning up was fine paul