First off, do not immediately jump to the conclusion that it is 'rider error' and exclaim 'You did it.' without supporing evidence.
Secondly, how'd he shift into the spokes with the spoke protector still on the bike? Certainly looks like a case of chain pin faults catching the derailleur. With less than 300mi on the bike, sure isn't much distance for something like that to happen out on the road. And not much time for the user to jack with the limits to be at fault.
My suggestion is to write out and detail everything done with and to the bike from date of purchase. If you can document the milage somehow prior to failure, all the better. With less than 300mi on the thing there's probably plenty of parts on the bike that will display very low milage. Document any work you had the shop do, and of course anything you did. If you worked on the bike, then I'd sure state how much experience you have working on derailleurs to support your ability to make such changes correctly.