@KOM:
If he was the victim of a DNS-AMP attack, nothing would change just because he changed DNS on his gateway or client, and any of his LAN clients doing excessive lookups also wouldn't care which DNS server was selected for use by pfSense. There was something weird going on but I doubt it was a DoS attack.
That's the "stopped just by coincidence, but that's unlikely" part.
Given that, it's more likely some client on his network is issuing queries that cause some remote server being targeted to send or receive a large amount of traffic. TWC's DNS server takes part, Google's much better than most if not all ISPs at limiting the impact of or blocking DNS amplification.