Final-year IB daughter asked if a classmate, a boy, can study at ours on Saturdays

Our daughter is in her last year of IB. Last night she asked, very casually, whether a classmate could come over on Saturday afternoons so they can work through physics together. A boy. She asked in front of my partner, so the moment was clearly planned. I said yes before I had thought about it. I don't object, and I'd much rather it happens at our place than somewhere I don't know about. But since then my mother has been on the phone asking whether the door will stay open, and now I'm unsure what the rule should be, or whether making a rule at all tells her I don't trust her. What did other parents of 16 and 17 year olds settle on? Or did you not settle on anything?