Oh, it's obvious. I've got plenty of pictures and videos. All of the breaks involve either the fence post AND the cement footing or just the cement footing going thru the pipes. A fourth post being dug actually push the pipe inward from the middle so the pipe itself had a slight bow. There is absolutely NO WAY someone digging didn't notice this. There had to be water seeping in those holes. My guess is they dug the hole for the fence posts, said "oh ####" and quickly covered it up.
Also, the City inspector came out several times and verified the damage himself. I have the reports where he specified it in the report.
If that's the case, I'd absolutely do what I could to go after the contractor's insurer. (if they had one). Again, I think there's a good chance you'll be out of luck (because so many distressed contracting operations carry lousy insurance or none at all) but it cant hurt.
And at this point, its in your neighbor's best interest to tell you who did the work. If you have proof that the damage was the result of their fence installation, and they refuse to give you the necessary information to make a claim against the contractor, you're obviously well within your rights to go after them (the neighbor) directly. That's kind of the whole point of requiring a contractor to provide a Certificate of Insurance. It's the only thing that keeps the client from being liable for damages they cause.
If they still refuse to give you the name of the contractor, I'd be willing to bet the contractor was uninsured. They probably told your neighbor something like "Yeah, I could get you proof of coverage, but it will cost you an additional $XXXX" (probably what it would have cost the contractor to get some ####ty policy and pay the min. earned premium before it went into cancellation) and the neighbor probably told them not to bother.