She has bipartite sesamoids, which happens when they don't fuse together correctly. That unfortunately means that it's highly likely to happen again. On the positive side, it's something that proper orthotics or padding should let her fix.

I started to get a little plantar fasciitis from too much mileage in my racing flats and excessive downhill running, but was able to fix that by rotating my shoes more and sticking to the flats.