You know what they say - start with a compliment!
I'd probably say something like:
"That's really kind of you to watch out for your friend! Unfortunately I have a very limited amount of space and I don't have the room for non-students in the classroom, plus I find that tends to cause distractions. If you'd like to stay for the first 15 minutes or so and help your friend get settled I can make an exception for that. Don't worry, I promise your friend will have a great time and make new friends in my class, I keep everything very friendly."