Wow, could I rant on this one. So if it gets long winded, sorry in advance. I tend to agree quite a bit with Maura's scale. If it is out of pain it is one thing, but this sounds like another thing completely.
I have been training horses for 18 years, have a mom who is a retired trainer and a grandfather who is a retired trainer. I WOULD NOT TAKE THIS HORSE IF THEY GAVE HIM TO ME.
I have re-trained a couple horses over the years that would rear, but nothing near as bad as this one sounds. It is not fun, easy or enjoyable in any respect. I would gladly take any bronc type bucker, bolter, charger, biter, kicker before another who rears. Some can be rehabbed, but most tend to revert to that as their escape method when in an uncomfortable for them situation regardless of training/fixing time. A dear friend of mine will never have children from one going over on her so I have a huge distaste for a rearing horse. If a client asked me to take on a rearing horse that had already killed someone, I would 100% turn them down.
A 16 yr old, REGARDLESS of how good of a rider is, cannot possibly have logged enough training hours to deal with an issue such as that. I had already trained a handful of horses from start to finish at that age and I would have never considered one that had that problem and my mom most certainly would have stepped in had I thought I was "big enough" to deal with it. SHAME ON HER PARENTS for even considering it. They are giving their daughter permission for e.r. Visits and possibly worse given he has killed one owner already.