It might be a good idea to buy a non contact volt pen so that you can check for internal cable breaks.
Those devices are unreliable. He has a multimeter that will check for breaks.
I've said it before and I dare say I'll say it gain but a hefty proportion of my fault finding is done with LED (often used in voltstick mode) and neon screwdrivers.
They have their place and if one bothers to learn how to use them and interpret results they can provide some good results and save a lot of time.