If he is filling to the MAX mark with a cold engine then leaving the cap off it will easily overflow.
This is one of the reasons it has a MAX mark, and it assumes that people leave the cap on the tank, as this is what it is for.
A corvette I fixed years ago had a HEADGASKET GONE according to CORVETTE SPECIALIST near yarmouth, so owner was told it would help if he removed seal from rad cap.
Long story short, I put a new cap on and cured the BLOWN HEADGASKET!!
One of the other reasons the water is rsing more is because of CAVITATIONas the air thus created pushes the water away as it has little resistance due to not being confined, normally this air laden water disperses into the tank, air is compressible water is not, this is alos why there is air pressure in the tank when you release the cap.
tabetha