I never would have sent to auction though. I would have either rehabbed if there was a chance of recovery or put down if there wasn't. No need to pass an issue like that on to someone else.
This horse needed to be PTS, there was no passing it off to someone else.
If it went to auction, it went to slaughter.
Op, I am sorry about the horse, it sounds like you really liked him, I would suggest working with someone else's horses in the future, someone with a sense of responsibility. I suspect if they treated their horse like this, they didn't treat you much better.