Isn't the "white line" merely the spray where the actual wave is swelling and tipping?
I got the impression that these are not like "waves" as we in this country think of them.
I suppose you could say that, and , of course, the guy isn't 6 foot but 5' 10".
If the distance from the camera to the people is 1/2 the distance from the camera to the waves, then the waves exceed 25 feet. If the distance is 4 times, then the wave height exceeds 50'. The truth is probably in between.
We need more information to really define the height, for example, is it a telephoto lens, the height of the photographer above sea level, and the height of the people in the pix below sea level. None of this takes away from the fact that it is one hell of a big wave.