Reddit, any creative ideas to keep people out of your bed?

My my friend is facing an odd situation. Next week, she is leaving her college for the weekend to visit her parents. Her roommate (who is also a girl) asked her if it was ok if a friend (who is a boy) came for the weekend (the same weekend she is leaving to visit her parents). Her roommate said that he was going to sleep on the floor so she wouldn't have anything to worry about.

Now this would be all well and good, but the thing is that my friend's roommate, although not a terrible person, isn't the greatest roommate in the world. She is sometimes inconsiderate (sleeps all day, complains about the lights, parties a ton, etc.). So my friend is concerned that her roommate's friend will use her bed instead of sleep on her floor.

My friend asked me what she should do to make sure this wouldn't happen, and it's the same question I am posing to reddit. I said that she should take her sheet home with her, but she doesn't really want to do that. I also proposed the idea that she take a tampon, pour ketchup on it, let the ketchup dry, and put it in her bed before she leave. She wasn't thrilled about that idea either.

I'm looking forward to some pretty creative and pretty useful ideas.