If I'm reading the OP correctly, this woman has accepted three separate, different jobs that all involve shift work or being on call. And her husband also does shift work.
Child care would most likely be an issue in that situation, no matter how hard the parents look for help. Just putting the kids in regular day care probably won't solve the problem here. So I can understand why the OP's friend is relying on others to help her out.
But she should still be trying to hire someone who can be on call, and have back-ups for that person. Relying on her friends is going to lose her a lot of friends, or at least a lot of the goodwill of those friends in a short time. Granted, her pool of potential care providers is smaller than usual, because of the demands of both shift and on-call work, but there are people out there who can do the job.
The friend has options. She could have searched for childcare before starting her new jobs, so that she could have the care provider watch the children while she was still at home, and thus get a chance to see how the provider got along with her kids.
She could start looking for a job with better, more stable hours. She could drop the most problematic job. She could ask around for trustworthy childcare providers. She could offer to pay one of her friends, currently providing care for free, to be on-call in the evenings for child care.
As for boundaries--I'm in a slightly similar situation myself. Since Monday, I've had my nephew and niece at my house, while my brother and SIL take care of my other nephew, who needed major surgery with only 4 days notice. The hospital is here, near me, but 500 miles from their home. Nephew and Niece are on March vacation, so they aren't missing any school.
But last night, my brother mentioned that their plans for next week, when Oldest Nephew will still be in the hospital, and the younger two will need to be back at school, have fallen through. The family member who was going to take care of them has had a health crisis of her own.
I'm pretty sure that DB would love for me to jump up and volunteer to get the kids back home and stay with them out there next week. But there are many reasons why I don't want to, most of which boil down to the fact that the kids, while great kids, are exhausting to watch and a huge responsibility and after this week is over, I'm going to be Tired Out. There's also my ancient car, the 8 hour drive in snow season (8 hours with a tween niece who cannot stop talking), the cost--just the gas and tolls for the round trip is about $150.
So I've decided that if they *ask* me to drive the kids home and watch them there, I'll do it. But I'm not going to *volunteer*, because if I do, even if I say I want to be considered only after all other attempts have failed, they'll stop looking for someone else and I'll be stuck. And I am only going to do this because he's my brother. For a friend, however good a friend, I'd be sticking with the one week I promised at the start.
And yes, I feel a little guilty about this. He's my brother! This is a crisis and I should be helping out! And it really, truly is a crisis. I have no doubt about that. I just keep reminding myself that taking care of two children for a week isn't "nothing." It *is* helping out in a very big way. And that in other medical crises (unfortunately, Oldest Nephew has several serious, on-going medical issues), I have gone out to their home and watched the kids for two weeks at a time more than once. It's not as if I never help them out. And it's not as if this will be the last time I'm asked to help out. The chances are high that there will be another call for help within the next year.
Just as the OP should remind herself that she has been helping her friend out, quite a lot. But it's still okay for her to decide that the amount of help she's giving has limits. Limits that work for the OP and her family, not for her friend and the friend's family.