Ok since you asked WWYD...
You did exactly what I would have done and picked up the slack for these idiots and you did right by the horses. They were very lucky that you and your DH (gem gem gem) where there.
Although at this point I wouldn't be able to just 'Let it go'. I would have to at least try to let them know that what they did was completely unacceptable. If not in best interest of the horses, then at least for the poor kind soul who agrees to 'look in' on their animals next time. I'd also be really curious to find out why they thought it was okay to leave their animals like they did.
I am about the most non confrontational person you could ever meet, but if you trigger one of my hot button issues - pets, children, or the disabled, I will take issue. What they did was just not cool.
This wouldn't be something I would do via text or phone, this would have to be a scheduled face to face. I'd schedule a coffee or lunch with this gal and I would let her know the agenda up front.