Loss of coolant leaking radiator leading to white smoke

Vortec engine. I had a radiator leak, the guy drove it after it overheated. The dash gauge said "check gauges" The gauge says red all the time even when the truck is off. Turn back on the truck has not even reached operating temperature and the gauge still is in the red. He changed the radiator but might be too late. There is white smoke out the tail pipe and it appears to be leaking coolant from the edge of the muffler itself.

Mechanics are saying to replace the entire engine. But, they have not inspected the vehicle just off the back they are saying to replace the engine.

Can it be a blown head gasket? Or is it possible the block faces are warped or the cylinder heads are warped. Or if there are any other possibilities can anyone share?