I mean at least you own up to the fact that you don't understand back pain. Figuring out where muscle pain comes from is actually pretty easy. Shit, anytime I have back pain, the reason why it hurts is easy to hone in on. That's why posture is so important. You should read up on back health, it'll probably prevent problems of your own down the line. Take care of yourself.
But there's an easier thing that can be done here to simulate the problem. You say that tits don't cause back pain. Okay. Pick up something that weighs 15-20 pounds and hold it there for as long as you can. After a while, your arm is going to start to feel discomfort, isn't it? You'll feel as though your arm is naturally going to want to be lowered because your arm muscles are getting tired of the strain. 15 pounds isn't a lot on a relative scale, but picking that up and holding it in place IS going to start to wear on you over time. You might think that because they're arm muscles they operate differently than back muscles, but they fact is they operate in the same way. Muscles are muscles. So Codi has 15+ pounds strapped to her chest at all times. If she keeps her posture up straight, which everyone should be doing (barring unique health issues), that means it's causing her back muscles more effort to do so. She could hunch over to give her back muscles a rest, put her tits down as it were, but again, good posture is good for your back health and you don't want to be hunched over. It's sort of a damned if she does damned if she doesn't situation for her.
Look, this whole thing sucks. It's a shit situation for all of us and none of us want this. I hate that I may very well find her unattractive after this. But it's pointless to be bickering over petty nonsense because none of this is going to change the outcome. There is no changing the outcome.