second hand account, and sometimes people say "smaller cog" when they actually shift to a LARGER rear gear.... I've heard that many, many times... and quite frankly, the damage level talked about could only come from the der. getting snagged in the spokes, or an overshift/tangle( i hate it when that happens)... and then, there's that earlier wreck to consider... bent der. hanger would be a result of the wreck, perhaps...

and since rider claimed to have been in the middle chain ring, with no reference to whether the front, or rear der. was shifted from the OP author, my assumption was of improper terminology as to the gear change......

more info, from the rider, is needed? Or a good inspection in person, which is not an option on these forums, eh?

and to the OP... i'd be looking at the spokes BEHIND the rear der...... overshifts cause hidden spoke damage that may only be visible once the cassette is removed... and inspect the frame for overshift off the small side, too.... once again... more info is needed.