Ok let me set this straight...
You are correct about the air going through the AFM and the ECU running the engine rich for a few seconds... However it isnt a myth that inlet reversion or compressor surge kills turbos...
Reversion of air pressure back across the turbo acts against the compressor wheel and causes the turbine to reduce speed quicker than it would naturally, this can often cause stress on bearings, the shaft and the wheel itself.
Also when the throttle is opened again, the turbo will have to make up for lost momentum and will take longer to achieve the required speed, hence more lag.
If your BOV is not set up proportionally to the pressure inside the inlet, at the point when the throttle closes the pressure cannot reduce quick enough and wont be able to escape through the BOV quick enough... This means that the only way for the excess air to escape is back through the turbo charger.
Your friends Liberty turbo may not be damaged due to the pressure not being enough to damage the internals of his particular turbocharger as most aftermarket turbos are much stronger than OE ones - which break more easily.
Inlet reversion or compressor surge mainly happens when an aftermarket BOV is installed and not set to the correct pressure, this causes the "fully sick" flutter noise and is actually a combination of pressure being released through the BOV gate and the turbo compressor.
Now I sit back and wait for the arguments