Even if a player did that, why would your character believe him? I could walk up to the druids right now and say "Shamanism is the proper form of magic. I know for a fact druidism is not the right way to use magic." It doesn't matter because it's my character speaking, and your character listening. The fact that each of the magical sub-groups believes so strongly in their magic just further reinforces the fact that nobody would even believe this. I would actually have more of a problem if somebody went around saying this, and the different magic groups actually believed him in-character. Your character has no reason to believe what this random person is saying, the druids have spent hundreds of years with these beliefs, why would they just suddenly change their ways based on some random hobo walking into their grove and proclaiming that "All magic is the same!" That's like saying you would walk down New York City streets and believe the guy with the cardboard box hanging from his neck screaming.