You were commiting fraud. He did nothing wrong by reporting you. Don’t want to get reported? Don’t commit fraud.
I’d agree if they were colleagues/strangers. I would never report a friend for that. I’d voice my opinion at the most but reporting them is effectively nuking the friendship.
This guy was previously admitted by a mental hospital and was up for mental evaluation. Proper gun control would mean he wouldn’t have been able to buy that rifle.