Still gotta say no. "More Wear" is not a new clutch. Like I said, offer to cover labor if the guy wants to buy the parts as a show of good will. Or alternatively, pull the clutch out, mic the clutch disc thickness and pro-rate the cost of a new clutch by the amount left. I suspect, at 8,600 miles, even ridden for a bit buy those idiots that clutch disc is over 80%. Remember these are stock clutches that are routinely slipped to maintain traction on standing, 5,000RPM starts.
But as a guy that races cars, and has raced Mustangs, and has actually used up multiple clutches, they didn't run through that much material.
What those employees did was disrespectful and shouldn't be tolerated, but they were fired. The work that was performed (to standard apparently) was done for free. Owner is asking for unreasonable things, and trying Social Media blackmail and threats of whipping up a mob to try and get his way. That's unreasonable in any light.
Don't get me wrong, I'd be pissed. And something like Mike's Grandpa's "free oil changes while I own the car" is probably reasonable to ask for. Were it my car, I might very well pitch the "I'm going to buy a new clutch and you're going to install it free" line and see if they bite. $8,000-$9,000 worth of parts and service paid for cash at another dealer? He's on crack.