In the end it doesn’t matter. You provide the feed, and unless there is a specific agreement that the BO will let anyone know when their feed/supplements/medications are running low, expect to take that responsibility onto yourself.
I’m not really sure what you are looking for in continuing to say “she always did it before, she didn’t warn me early enough this time”, when she DID give you notice and you chose to not make sure there was more feed before it actually ran out. That’s on you. I can’t imagine anyone at your family gathering would have had a meltdown if you’d said “I need to run out for a little bit so my horse doesn’t run out of food”. Was the feed store also closed on Saturday and Sunday?
Did you ask her what “running low” meant when she let you know on Friday? If you didn’t, and assumed that meant a week left, that’s on you. If she told you “a few days” and you still chose to use company as an excuse, that’s on you.
Life changes, people change patterns.
We are responsible for how we react to events that happen.
You finally got “bitten” by something that isn’t even new. The first time you got short notice, that should have been your cue to take the responsibility for making sure there was always feed in the bin.