He did actually tell her to put the horse down. Even if she thought about gelding him, they would have had to use a tranq gun, because there was no way they'd be able to get close enough to give him a shot. Not to mention then trying to make sure the wound heals right. While I do think that a lot of "dangerous" horses are fixable, every once in a while you do find one that is not, and it's much safer for everyone involved to just put the horse down.
That being said, I really like Buck. His approach is more common sense, and understanding herd dynamic, then gimmicks, games, or a system that you have to follow in order to get results. To me Buck is not a showman who's trying to say his way is the best, or the only way to get results. He isn't trying to sell a product, or a system, or a "magical" bit or other equipment. You can implement his techniques along with other trainers to reach your goal, it's not like say parelli where it's a whole system, and you have to stick with a program. At least in the video he isn't wearing logo clothing, he looks like a working cowboy and he's almost always on a horse, showing how to do things. He doesn't take horses, work with them for a day, then hand them back to the owner, he corrects the owner, and helps the owner. No matter what discipline you ride, his methods work. Someday I would love to go to a clinic of his.