So sorry to hear about that happening, and I think you did your best. In my opinion, you went above and beyond to try to save the boarder's horse.
I have been places where the BO did everything they could for a hurt horse, whether it was their own or not. I have also been to barns where they don't care nearly enough. Trust me, better to care. :)
Unfortunately, these things happen. I don't think you are unlucky, I think whatever is going to happen is going to happen. A couple years ago we had two horses put down in one day. One was young, too. The other was a beloved lesson pony. The pony coliced and was so bad the vet advised just to put down, he wasn't going to recover. He was old. The youngster had a muscle deteriorating disease, so while the vet was out we decided to just have him put down too before he got too bad to where he couldn't stand anymore. At least end his life while he was happy, and not in pain.
It was a hard decision, but one the BO made, and knew she had to make. It sucks, but such is life with horses.