If I ever saw a horse rear that high, no matter what the circumstances, I would run, not walk, in the other direction.
Rearing is not easy to stay on. If you get surprised, you can end up pulling on the reins to hold yourself on. That in turn can bring a horse over backwards.
If you are not strong enough to hold you upper body in position, you slide/fall of backwards. Doing that can easily break your back. If the horse falls over on you, we are talking about the possiblity of broken back and a pelvis in pieces.
When I lived in Ky, I was told about a gorgeous horse who had sold for $1,600,000 as a yearling. I went running over to see him. Oh MY! He was a vision and a 10 mover. His price was $2000. They got an exercise rider to get on him and trot him around on the training track, near the gap where horses enter and leave the track. First time by, he got pissy about not being allowed to go through the opening. OK, he was surprised that he was being ridden there, in a circle. But, the second time around, he stopped, backed up and reared straight up.
By the time he came down I was walking away. No horse in the world was worth getting hurt.
I heard several weeks later that he had reared again with his exercise rider who had come off and broken a bone. I was very glad it wasn't me.