Just tell her that you understand she wants to stay to make sure you are settled down and that it's a long journey home. Just say "The university suggests our parents don't stay with us, as we need to bond with our flatmates". Maybe you could then offer to meet the morning after for breakfast, tell her all about the night before, and then see her off? Don't feel like a terrible daughter. It seems she is trying to guilt trip you here. If my parents said they wanted to stay when they drop me off in September, I'd just laugh and say no.