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.
Sounds like it had a platform of plywood. Had to be at least an inch thick to hold the weight of an adult (if that was his goal.) That would be quite a bit of weight in itself.