I'm not saying that she should go in "guns-a-blazin". No matter who these people are, she needs to stand up for herself and let them know she is not happy with not getting the service she paid for. It sounds a little like you guys are intimidated by them - Don't think of it like high school where you don't want to offend the popular kids for fear of retaliation. At least one of these ladies knows the complete arrangement, and she needs to communicate it fully to everyone involved.
The meeting is for clarification, not to yell and scream.
For example, if I was in this situation.
Friend 1 - Hi
Me - Hello. My horse was supposed to be turned out on x days. The horse did not get turned out. Additionally, what's the deal with friend 2's horses? Am I supposed to be taking care of them as well?
Friend 1 - Oh whoops! I'll let friend 2 know. Yes, please take care of friend 2's horses.
Me - Okay, but what days are you taking, and what is friend two taking? Because doing additional horses increases my workload, and it's only fair we all pitch in.
Friend 1 - oh... well. Here's what we will do: blah blah blah.
Basically, this is a business arrangement. Don't think of it as being nice to friends first, because that's how you'll get taken advantage of. Think of yourself as the client paying for these services with your labor, and if they are not providing what they promised, they need to be fired.
You guys won't automatically end up enemies just because you can't agree on days. But don't let them take advantage of you, because you are setting yourself up for them to continue to take advantage of you. It's just like training an animal - let it get away with something, and it has no incentive to stop the behavior.
What would you think the politician is going to do to you? You're not doing anything illegal or bad or even mean. And if he's worth anything, if his lady comes to him complaining that someone at the barn wants her to stick with her promise to exchange services, he'll tell her she should stand by obligations.