Every coach at every level has strengths and weaknesses. Every coach has something good to offer, regardless of method of delivery. Have your player seek out the good that each particular coach is offering. Help your player understand that each coach will also have areas that are not his particular strength (just like us parents). Always have respect. Understand that there are often more than one right ways to do things. I can't emphasize this enough. Yes, there are coaches that are coaching things that are just wrong but, even more often, I see kids/parents furious and adamant that their kid is being taught incorrectly when, in fact, it is just a different right way.
So, with the coach that is particularly low on the game knowledge aspect...
COMMUNICATE. The majority of those coaches have at least some idea that they are not master mechanics of the game. If there is a distinct difference in a mechanical or philosophical teach and the player is conflicted, he can politely ask the coach about it after practice or during a break. "Coach, can you help me? I'm sorry but I'm confused. I have been taught XXX. Can you explain why XXX is wrong and we are supposed to do it this way?"
Both will probably learn something and it is just the right way to handle things.
I absolutely hate the idea of telling a kid to nod his head in agreement and then ignore. What are we teaching them? I have seen that blow up and cause more problems so many times.
Even if you are producing, I would rather see it play out where you tell the coach in private, "I'm really struggling with this. I have always been taught something different and I am having a hard time adjusting. Is it OK if I do it this other way instead if I know it is what works best for me?" This is better than nod and ignore. If, then, you still get a completely ridiculous reaction, you know what you are dealing with and you just play. But be open to actually listening to what he has to say. Again, both can learn something.