Take them for a short hour sail, as long as it's not averaging more than, say, 10-15 knots wind. Tell them the head is broken. They never need to leave the cockpit. Unless you're experiencing delamination, I'm sure it can handle the weight, even if they snuggle. (I worked for a Catalina dealer and know from experience that they are very sturdy boats.) Tell them where to sit, out of the way. As long as they can board and un-board without trying to pull their weight on your lifeline stanchions you won't likely have any damage. (Point out that most people, sailors included, don't realize that lifelines are designed to function more like curb feelers than guard rails.)
I'll also second Tom J's comment that the boat captain has absolute authority onboard, but that's relevant to all passengers, not just heavy ones. If they can't agree to that, all else is moot, cause they ain't coming aboard.