My mom had a boxer (who is 70lbs) who was never consistently trained, she wasn't mentally sound but consistent training and dominance establishment would have considerably helped that. Anyway, one day (when we were still living with my mom, it was almost 2 years ago) she was asleep on the couch and my 20lb german pinscher came in from outside, was all rowdy and she went and tried to play with the boxer. Well, my mom's dog flipped, grabbed my dog by the throat, wouldn't let go and started shaking her. She let go for a second to get a better grip and I was able to push my dog away and hold my mom's dog back. My dog had to get 8 stitches: she had a long tear on the side of her neck and two puncture wounds in her throat that went through the muscle. The vet bill was very high because it happened at 8pm so we had to call an emergency vet.
I was seconds away from running to the kitchen to grab a knife. If my mom's dog hadn't let go when she did, I'm positive I would have killed her. I've never had that thought about a dog before EVER but when something you love more than anything (this was before Callum

) is being killed (and my mom's dog was attacking to kill), thoughts go through your mind that you never thought possible. I'm glad my OH wasn't home because he would have killed my mom's dog, no question about it.
What I'm going about the long way of saying is, FOB can say that he would kill a dog that attacked Percy, but what if Percy was the aggressor? What if Percy killed someone else's dog...or hurt them badly? What if someone reacted like my OH would have and kills Percy to save their own dog? A dog that thinks it's dominant can be a very dangerous animal. It's not responsible pet ownership to get a dog and then not work to establish dominance or train it. It's not fair to the dog, either.
Good luck, hope you can talk some sense into FOB and I'm looking forward to seeing the new pictures of Percy.