Some physicist needs to figure out based on how high it went if it could have lifted off with the 50 pound load.
I'm guessing that there's more than a few people at the nearby Air Force Academy that could figure that out very quickly.
That is so dependent on the temperature and the volume of the helium and that it is nigh impossible to be very accurate. The balloon was pretty floppy much of the time- makes it even tougher. Also who knows the weight of the “gondola” structure... the mylar is nigh on weightless. I’ll take a crack at it later on. I believe it very capable of lifting 50#, on first guess.
If I recall correctly a cubic foot of helium can only lift about an ounce of weight. So to lift 50 pounds you would need 800 cubic feet of helium. If the balloon was a 20 foot diameter circle, it would only have to be an average of 2 1/2 feet thick to hold 800 cubic feet of helium. It was fatter than that. I assume the balloon itself was made of very light materials. Of course you have to correct for the altitude of Denver, but basically it looks like the balloon could have lifted the kid.