Put up reminders around the barn the day before.
Maybe write it out on a dry erase board and attach to the front of his stall.
ALSO, leave messages for ALL the responsible parties
(ie. text messages, emails, Facebook, post it notes etc.) so that NO ONE has the excuse of not getting the memo.
Sadly, usually,..., if you want a job done right, you have to do it yourself
Doesn't really seem right, but people do forget, or resent being asked to do 'above and beyond' their usual duties.
Be optimistic,.., Go into it with the idea that she will remember,
and you have preempted the situation with enough notes, messages, etc.
Hopefully, things will go as planned
I do not think you have any grounds to make anyone else pay if things don't go off accordingly.
Probably, you would have to just reschedule the procedure.