No, the horses weren't traumatised. As Kristy said, they don't have the mental capacity to understand what happened. My husband has had to do the same thing and the horses were fine. He took the mare to the spot she was to be buried and did the sad deed, but, there were horses still around, when ever they see us they will follow us for the most part. They horses were probably startled by the noise. We've also had to kill animals that got into the barn (opossums, racoons etc) and the horses "jumped a bit" from the noise, but a minute or two later were fine.
I think the owner did the responsible thing by putting the mare out of her misery. Too many people are unable to do it and utlimately the animal suffers.