Carbon deposits can build up to the point where they retain heat from combustion and "glow". They continue to glow during the compression stroke and when the cylinder pressure reaches the right point, the glowing carbon can initiate combution (before the spark plug). This is "pre-ignition", causing the pinging sound your friend hears.
The symptom can also be caused by having the timing advanced too far where the spark plug is initiating the combustion, but too early.
The pinging is actually the explosion of combustion hitting the piston while the piston is still traveling up in the cylinder. In a severe case, this can bend a connecting rod or "hole" a piston.
I assume that he has tried the highest octane fuel he can get? How about trying a can of octane boost? High octane fuel burns slower and gives the piston more time to reach TDC before the explosion meets it, curing the problem. The next step would be to have the ignition timing checked.
I hope this helps.
Please let us know how it turns out.
Experience is something you don't get until just after you need it.