But boiled water (gas) is ineffective at removing heat from the cylinders. You have water, but it's not cooling.
Well, tell that to all the old car guys that have had to do it to limp home.
BTDT.
If you allow it to pressurize, it WILL blow out the liquid.
If you don't, and the engine is not being worked hard and going over 212°F, then it will not boil out and turn it all to gas / steam vapor in the time it takes to get there. If he was driving for HOURS, I would maybe think differently. He's driving maybe 15 minutes at a time.
Let's assume for a moment that he has a hose with a hole in it down low on the engine.
If the cap was on, it would pressurize, expanding the hose and push out the coolant.
If the cap was off and it wouldn't allow it to pressurize, you will lose SOME coolant, but not all of it, and the hose will not leak because it's not being pushed open.
Just like all the old cars did not have a coolant recovery system on them, the capacity was oversized so there was always some "air gap" in the radiator because as it warmed up, the system would push out and "excess" coolant, then it would stabilize because it would not expand any further. Just like this will do. The coolant (water) will expand some and will allow some (maybe a quart) to overflow out of the radiator, but it will not PUSH it out unless he has a massive head gasket leak where the cylinder it truly pressurizing the system. Then it will violently puke coolant out of the radiator. Clearly he does not have it that bad yet, but he's still trying.