Maybe it is just the angle, but why are the kids jumping out those windows and not the ones that seem to be directly over the mattresses? I guess this shows my age: my first reaction was to ensure these kiddos were safe! It does look wicked fun!!
You say that based on Doppler effect or you have more frames?
He's clearly facing a diagonal that will lead to the mattresses. People usually jump on the direction they are facing, but there isn't enough information on the image to tell if he's doing the things people do most of the time or if he's laughing after jumping to his death in a very unusual position.
See? You are the one I want to have around when I'm doing these kind of things. You are completely right, those other windows aren't as easy or safe! I just didn't notice and would have fallen to my death with the sash most likely. Having smart friends who think more clearly than "yes, let me do something dangerous for fun!" is awesome and the reason I have to still alive.
I suspect they jumped from one more floor up and pushed off hard toward the landing zone. Ballsy maneuver either way, you slip wrong on that jump probably your last.