I can find no other way than to say this is misinformation!
First of all, corrosion inhibitors are not the only reason for using antifreeze. The most important feature is the chemical composition prevents freezing at temps way below where water freezes. Water boils at 212*F at sea level, while anti freeze boils at around 230*F at sea level. By mixing it and water and putting it under pressure we can get to almost 270* F before boil.
So antifreeze does increase boil over protection.
Pressure increases boiling point. The formula is about 3* per pound.
Second, the flow of coolant into and out of the recovery tank and radiator are not as you indicated here. As the coolant heats up it expands. as it expands, it takes up more space. If it has no more space to take up, it builds pressure as it expands. Once the expansion pressure overcomes the cap's pressure, the cap lifts and the coolant enters the expansion/recovery tank. After engine shut down, the coolant cools and shrinks. As it shrinks, it creates negative pressure, or vacuum. This draws the coolant back into the radiator from the expansion tank. The bottom line is the coolant moves from the radiator to the recovery tank as it gets hotter, not colder and moves from the recovery tant to the radiator as it gets cooler.
If the water in his radiator is 220* when it goes into the expansion tank, it WILL boil because it has exceeded its boiling point and is no longer under pressure.
While, not being there to see this with my own eyes, I think he is experiencing normal behavior for the conditions stated and people are going to have him throwing parts and money at a problem that doesn't exist.
This can be tested easily enough by draining and flushing the system and putting the correct mix of water/antifreeze. My money says he will no longer see boiling in the recovery tank.