[olddog]

I would have guessed Laughing gas and blown as my second guess.

Back in 89 to 93 years, Ford commanded about a 10:1 AFR at WOT. Chip makers were commanding 12:1 AFR and getting about 30 Hp gain. They stated must use premium fuel with the chip installed. Those who didn't read and head often ended up with pistons like you pictured.

In the ovoid detonation world of tuning, pulling timing or going rich is the only tools. Timing kills more Hp than going richer. Obviously fuel and combustion chamber matter but the tuner has no control over those. My guess is that the tuner tuned the engine with a good premium fuel and the owner bought low octane fuel. He didn't hear a ping and thought he got away with it, and he did for a while. Now he thinks it was the tunner's fault. The tuners should make you sign a paper acknowledging that you were warned that you must run a premium fuel.