I have neither shipped eggs nor had them shipped to me, but I've read LOTS of threads like this, and it's amazing to me how people want the egg producer to take responsibility for incubation problems that are far more likely to be on their end than the producers. Good grief. She received fertile eggs in good condition that developed clear up to the hatching stage... what else does she expect you to do? Stand over her incubator and make sure she's doing it right?
This is why I am terrified to begin shipping eggs, even if I didn't have more than enough demand for them right here locally.
People are nuts.
Luckily, all my hatching egg customers have been experienced and very very nice about everything, and they've all had good experiences. But the truth is, I get 80-90% hatch rates with my eggs, and most of my customers get closer to 50-60%. I say that's *their* problem, not mine (and, fortunately, they agree and have NEVER complained--they all seem happy with their hatch rates, so it's all good). Problems that develop late in incubation are rarely a problem with the eggs themselves, unless you have weak stock or disease in the flock, but in that case *you* would be having trouble hatching too.
Good luck, and I'm sorry you have to deal with that. I hope the customer is not being too ornery, but don't feel bad about it anyway. You did your part.