So you can diagnose what caused the frame to break from a couple of pictures and an internet description? You're good...you should work for a dealer. And you seem to have the moral high ground covered as well, it must be very satisfying pointing out how others are being dishonest from your ivory tower.
It's not your job to pass character judgement on actions or recommendations, nor is it your place to determine causality for the problem at hand. I'll make it as simple as I can for you, the grinding didn't cause the break, and while the improper installation may have, that's for the dealer to determine. I offered my advice because of my experience in the field. Either the OP will follow it, or he won't. Have a great day.