Science can be a liar sometimes
There could be a number of different reasons for her to pull out so late. But, assuming her statement is the real reason for why she is pulling out, it's important to keep in mind that you may still see signs of a stress reaction on imaging long after the stress reaction has healed. She could feel completely normal and feel ready to go, but the image may still show signs of bone stress. So if her team really felt she was ready to run, they could have ordered the image to "ensure" she was healthy enough to run, but that could have also given them false feedback relative to how she was actually feeling. If you were to MRI the entire lower half (pelvis down to the foot) at any point in a non-symptomatic runner during periods of elevated training, you have a high likelihood of seeing positive findings that would indicate a stress response somewhere, but this doesn't mean it's an "active" fracture. So again, assuming her statement is accurate, her team may have made a mistake in letting imaging dictate the course of action rather than confirming a clinical suspicion based on symptom pattern and testing. Now if she had active pain or previous imaging with the same diagnosis during this build-up and they wanted to see progression of healing or a recent clinical suspicion of that injury but just hadn't received imaging sooner and was still experiencing discomfort though still running, that's a different story.
FWIW plantar fasciosis can cause stress responses at the attachment point, especially if accompanied by a heel spur. We don't obviously know where in the calcaneus her reaction appeared, but assuming it's related to the plantar issue she's been having, then she may see a hot spot at the attachment point (but that doesn't mean active stress reaction, especially if her plantar issues have occurred prior to this build-up).
FWIW x 2 - it's rare to order follow-up MRI imaging for stress fractures once the suspected diagnosis is confirmed because symptoms can appear on imaging anywhere from 6mo-1year post depending on patient's bone density scores. So if her team had previous imaging and wanted to track progress, I'm not sure what they were magically expecting to see that would have confirmed she was "healthy" enough to run if she were pain-free. Pain still should trump imaging findings, which is why I suggested earlier that although she may have been running and may not have felt pain, maybe she still felt something that also could have started increasing this week (or suddenly today) and hadn't received an MRI yet for the previous plantar symptoms (which would surprise me) or they wanted to compare previous imaging (that may not have showed a fracture but now did).
Just some thoughts