I bought a Beretta folding 20ga shotgun new in 1958 -- teenage purchase, the only shotgun I could afford -- and spent a lot of evening hours practice-pointing and dry-firing at imaginary birds who usually headed for one upper corner of my bedroom. On one trigger pull I heard a "click" from that corner of the room and saw something small fall down the wall and behind a dresser. I knew immediately what it had to be, found the piece and determined that it was indeed the front 1/8" of the gun's firing pin.
I was mortified, of course, because I had been told not to dry fire any firearm. But I set aside embarrassment, took it back to the owner-operated gun store where I bought it and confessed. The owner charged me a little under four dollars to replace it, and I never dry fired the shotgun again. (But I did continue to practice shouldering the gun and tracking imaginary birds breaking cover.)
So yes, I have seen dry firing break a firing pin. I did it myself.