Staying at an Airbnb in Spain for weeks. My friend wants to visit me for one night. Do I need to report him for the tourist registration thing?

I’m in Spain for a work trip, my Airbnb was booked by my employer, and I doubt he would be pleased with me if he heard I had anyone over. He’s quite neurotic. I’m worried if I contact the Airbnb host, it will get back to him, especially as there is a fee for registering a tourist, isn’t there? Could I just register my friend without involving the host? I’m not sure about the laws and policies concerning this.