I am not a professional trainer (and there are a few good ones here) but I would pass without KNOWING he had killed someone with the rearing/flipping. I am interested to see what other opinions there are regarding this. I know there are just some horses that cannot be re-trained from this type of behavior, and perhaps some that can. It would take a professional to even evaluate this horse safely and accurately. If your friend won't listen about passing on it, maybe you can talk them into taking someone with experience with this type of problem to go along.
"If you talk to the animals they will talk with you and you will know each other. If you do not talk to them you will not know them and what you do not know, you will fear. What one fears, one destroys." - Chief Dan George