I do highly recommend the small Champion book and card for reading plug condition. And please use a recommended tool to adjust gap, not as I saw quite recently, a plug clamped in a vice, and the "adjustment" being "achieved" with a hammer and cold chisel.
Such sound advice too, is a great tool for troubleshooting engine condition. For those that want to modernise their thinking on piston engine maintenance there are many resources available on the internet, APS being only one source.
As for the other, I'd be requesting he can't work on my machine, unless under direct supervision. That being my obligations as an owner in CASA regulations.
I always make sure to be shown the disassembled filter and strained engine oil from the engine and ensure that both engine screens and the main oil filter are serviced and inspected. CASA AWB 85-014 shows very good reasons to do so.