Rockwood water leak

My brother picked up a Rockwood tt a couple weeks ago. The main slide has water damage above the slide and delamination on the outside. Right above where the slide meets the wall, full length of the slide it feels rotted out when you push on it.

Is there any way to fix this? What's between the two walls? I told him to go trade it in fast and not deal with it. He's pretty new to the tt world and thought he ran up on a great deal. Thanks for any help...