Your use of the word "likely" here concerns me. Is this your diagnosis, hers, or someone else's?
I'm not going to recommend anything regarding these details, because I don't know what's actually going on based on this post, so keep that in mind when reading my later advice.
This sounds like a combination of weakness and control problems. I have never met anyone so weak that they could not learn how to move their body for these movements (if with assistance), so you may have to spend a good bit of time teaching her how to control her lower back. It may get frustrating and time-consuming, but that's part of the deal. Many people are bad at controlling their bodies, and you have to do a lot of work to get them to move correctly.
If you want to show us a video of what you're talking about, I'd be happy to take a look and give more advice.
This is a good indicator that this is a control problem.
Focusing on success for someone this weak is a good idea. She needs to be pulling with a flat back though to incorporate those muscles and get them stronger. Can she get a flat back in a rack pull? Hard cases respond well to rack pulls starting high with a flat back, then lower it and do a set, and keep lowering it and do sets until it's on the ground. This should all take place in a single session, but it works very well.
Cuing retraction of the scapula at any point in the deadlift is misguided.