I have no plans of ever having a horse flip over on me, thanks though. Perhaps, with so many horses rearing and fliping over with you, you should consider that there is only one consistant element. You.
I'm not saying that she should have to keep the horse. By all means, sell it. If a parent is silly enough to buy this horse later on down the road without proper information, or seeing it ridden, then that's a shame.
Before putting this horse down, it should be ruled out that the problem cannot be fixed. That's all I'm stating. A rash decision to put an animal down should never be made. If this habbit continues, then yes, I agree that it is a danger. But, a one time rear is not reason enough, to me, to condemn an animal to death.
Quite arrogant since I told you I fixed the problem. Also quite arrogant to say "so many horses" that have flipped on me. How do you know how many have flipped on me? I'm the one who takes the horses after they learn to throw themselves on the ground. I don't think you can truly understand how terrifying that is until you ride it, or how brave you have to be. I'm not bragging because I'm no big name trainer - But I don't tolerate that, and I don't expect anyone else to. If you can't fix it, get rid of it, before you get killed. You don't always plan to get flipped on, it just happens and sometimes it is nobody's fault.
A parent being silly enough to buy it...So sell it to the ignorant, and pray they don't get their kid killed? Great philosophy, not all sellers are honest, and not all buyers are born with the decades of experience it takes to recognize the signs.
There are too many factors, and I'm simply saying that you have no right to tell the owner she is making the incorrect decision when you know nothing about the horse. I've said what I would do, and I've said I understand if the owner feels that this is the best option.
Like I said, how about you volunteer to go save the horse? How about you find homes for all the non-dangerous horses too? I have an unstarted mare who was left unhandled for her entire eleven years. If you can find any market value for her, I'll be really impressed. You can't always play the self righteous. You can't always find the solution. You can't make everything black and white with horses. You can't make a decision based on a thread on a forum.