I think the walk-in sites are designated one-night-stands, so your friends would need to take the tent down. But it could just be loosely rolled up and thrown into the car, so wouldn't require much time.
Of course I could be wrong. I am sure it will be explained in the rules at the sign-in kiosk.
You could maybe try for a standard camp site once you arrive. Not sure if there is a campground host that has a handle on whatever is still available, but it might be worth the try.
