i can't for the life of me think how trail running damages trails any less/moreso than backpacking, although i'm open to other's opinions on the matter. as both a trail runner and backpacker i have some insight on this. when running a trail you carry substantially less weight, your shoes are lighter (less imprint), and your strides are generally longer. i argue i'm impacting the trail more when i carry 30lbs and wearing 3lbs hiking boots.
and wishing somebody breaks their leg is bad karma, especially for a hiker to make the comment. life has interesting ways of balancing things like this.
i wish him the best of luck and a good trip.