Most have to learn the hard way. Sometimes it is pure ignorance or being a beginner, so this is the only trainer they have ever known. Or maybe the bad trainer helped with one element of the person’s riding ( trainer X helped me learn flying changes! she is so amazing! no one else could do this!) Its hard to see the forest for the trees when you are ingrained in a program. What helped me leave a bad trainer is a friend who I respected started to board at the same stable and was able to lift the wool and point out all the inconsistencies and issues that I had been unwilling to see.
I think the easiest way would be to take your friend to audit with a GOOD clinician, where they can see what a good teacher looks like. Exposing them to as much education and other trainers as possible takes the blinders off and removes tunnel vision.