That would haunt me forever also. Especially now, as he is starting a new job in that area, and will pass buggies all the time now. There are too many of those accidents. People just get in a hurry. There was a buggy lane made on the hilly parts, but this must not have been one of those.
The images stay with you a long time. Worse because the horse lived while he held it, I bet.
A couple years ago I witnessed a fatal pedestrian accident - two people were hit by an oncoming car, both ended up dying, one immediately and one later, and I think it was a long time before I didn't see those scenes play in my head when I closed my eyes.
It has impact, and though I can't feel for the horse any more, it's too late for him, I can feel for those that witnessed it.