The honest and bitter truth is that your hunter sucks. First of all it's a well known fact that the belt can be taken care of by hunters with little to no failure. We've killed siegecrafter multiple times in both 10 and 25 man with hunters going to the belt and very rarely do they die during an attempt.
When they do die they usually have a good reason for it such as "I suck, and I fucked it up. Won't happen again".
But the fact that he can't do the belt is alright, some people just aren't the best players. The real issue is that he's trying to blame outside factors for his own personal failure. He doesn't know how to do it, he doesn't know how to improve, he sucks.
What I'm guessing was happening was that he was using the jump in pipe then disengage trick and if you hit disengage too early you hit an invisible wall that drops you back down on the platform, the problem is it often will shoot you back into the pipe or just you'll now be on the platform for another overload or two and take too much damage and die. That or as people guessed he was jumping ontop the pipe and simply fell. The jump into the pipe and then disengage trick imo is the safest and easy enough to perfect. Doing it that way you can take zero damage from superheated also.