Interesting business plan: Pa fixes amps while Ma demonstrates the refurbs and maintains a flow of prospective clients. I like it!
I don't think any repairer worth his salt is going to work cheap or spend the vast amount of time needed to teach you how to avoid his services though. You'll need to learn the electronic theory and design, components and practices of manufacturers before you can confidently repair anything you're likely to come across relating to audio electronics. You're also best off reading a general electronic repair guide text first, to learn why repair is not a simple undertaking and how long it's going to take to be proficient at repairing commercial audio products, acquire and use the necessary test instruments, identify faults correctly and get the repair/replacement work done without making a mess of it.
For a quick illustration of how basic solid state faults can sometimes be tracked down with visual clues, this video about the repair of a very simple stereo amp. should help with seeing how difficult it might get unless, like the presenter, you already understood how they worked, the likely component types, values and functions and how they're all put together, which varies significantly and becomes confusing with the changes in electronics that have occurred over the 60 years solid state home audio has been around. Some old amplifiers were built like birds nests of loose wiring inside, recent models with impossibly tiny surface mounted components. Nothing is universal or always follows the same rules.