If someone told me that they were allergic to rudeness, I would respond with understanding and respect. I might say something like:
"I completely understand. Being treated with kindness and respect is essential for positive interactions. It's great that you prioritize respectful communication, and I'll do my best to ensure our conversation remains courteous and considerate."
It's important to acknowledge their preference for a respectful and polite exchange and make an effort to adhere to those principles during the conversation.