if they got to shale beach then I'm guessing they at least got some pinstripping on the way. When I went out there with my daughter this year there were some brushy spots on the trail between the beach and the loop trail that takes you around the south part of the island.
Without talking with the guide, I wouldn't put all of the blame on them for body damage. If they said they wanted to test the rigs and didn't tell them that they wanted them in showroom condition when done, then he may have taken them where thought they wanted to go.
What I've found is that when you go out with people with no experience, they may tell you that they want to do some "tough" trails because they don't know what tough is. To outsiders, driving through a mud puddle can be tough or really pushing it; whereas to me it's not even beginner. Without good communication between the two, it's hard to say who is to blame.