Not a surprise. The demands of street driving can really tax an engine and the gasoline's octane rating.
On the track under real hard acceleration the ECU goes open loop and can fuel the engine richer than it would otherwise. This is to help satisfy the torque demanded by the driver as the engine makes more power being fueled richer. In this richer fueling condition though the engine is less inclined to detonate. Additionally the combustion chambers do not fill as much with air and combustion pressures are down. Ignition timing is not nearly as advanced as one might think either. While detonation is always a concern under these operating conditions it is not as much a risk as one might think. (Or more engines would expire on the track due to detonation as many drivers do not use race fuel while on the track.)
However, around town under low RPM and high load driving such as is encountered when cruising at lower speeds in higher gears the engine is run leaner, at the 14.7:1 mixture at which the converters are most efficient at processing exhaust gases. But a low engine speed at part throttle operation under high load conditions with the throttle butterfly valve open more fully the combustion chambers manage to fill up quite a bit with air and combustion pressures are quite high. Ignition advance can be as advanced under these operating conditions as it ever can be and detonation can occur. In fact much more likely to occur than one might think.
The next time the friend of your brother should get a dual tune: one for the track; and the other for the street. These can be selected by a toggle switch located somewhere in the car's cabin.