[SoCal LS-L]

The fact that it blew so violently without even being very far overheated, and as he stepped on the gas, tells me the engine compression was forced into the system.

I wouldnt drop the cash on a new radiator until you have the engine block tested by a shop (or do it yourself) or you risk the same thing happening again to the new radiator. A stuck thermostat will NOT cause cooling system pressure to blow out a radiator, the cap would relieve the pressure into the reserve tank. If the cap stuck, the coolant/water mixture would have to be SCORCHING hot to boil so violently that it exploded the radiator in that manner. According to the OP, it was "just a bit" higher than normal on the gauge.

Just sayin.... get the engine tested with a block tester or a smog machine to check for hydrocarbons at the radiator fill neck.