Having a Kerdi "card" or having a contractors license is no guarantee that the contractor you hire is going to be fair, honest, respectful or competent. Ask around for references, ask other tradesmen you trust, check around on the internet and ask around at the supply houses. You'll learn a lot more about this so-called card carrying "contractor" from others than you will from them.
Pay attention to how respectful this so-called contractor is of others, and you'll have some idea of how they will treat you as a client.