Was the horse tied up with a safety knot? If so the owner should have been able to pull it so the horse could have backed away safely. Or a safety link should have been used
I refuse to tie down my horse after experiencing a similar situation many years ago. I have heard horror stories like this way to often, so I purchased a tie ring that allows my horse up to 12 ft of rope if there is a panic situation. It works wonders! For example, My horse was tied with the tie ring on my trailer one day when A friend failed to secure the back of the trailer when she retrieved her horse. A huge gust of wind came and my trailer door flew open with a bang. My horse jumped back about 9 feet, accessed the situation and calmed immediately. I was about 20 feet away so a safety knot would not have helped in this situation, I would never have reached him in time. I have learned if a horse has room to react, he will usually calm himself down if he doesn't feel constrained.
My trainer always uses a safety knot, but I have a "stingy" performance mare that is so strong and does more than "set back", she panics, just like this gelding. She damaged her face once and the last time, she broke her coffin bone (yes! Vet= "a freak accident"!) I choose not to tie her and even though she dances around, no "freak accidents"!
This the reason why I only use full leather,single stitch halters on my horses. Multiple break points.