My ‘friend’s’ trimmer had stopped working, so I took a look. Initially, I checked that a new battery didn’t resolve the problem. This did not solve the problem so I took a look inside.
The internal circuit was quite simple, a sliding switch makes a connection between the battery positive terminal and the body of the motor with a wire returning to the negative terminal of the battery
Normally I would use my multimeter to trace through the circuit but in this case, it was possible to do a much simpler test, touching the battery on the body of the motor and using the wire to the negative end of the battery. The Trimmer motor immediately worked, which suggested the problem was in the switch.
Inspecting the switch more closely showed the contacts were not making good contact with the motor body.
This type of problem can be easily fixed by getting bending the contacts on the switch to increase the contact pressure with a small pair of pliers. This type of fault is common in similar mechanisms.
I then re-assembled the trimmer and it worked as it should.