Is the question whether a single spoke failure can cause an endo, or whether it was the likely cause of one in the particular case?

If the second, the evidence exists to answer the question. If you draw a free body diagram of the bike and rider, properly estimating the center of gravity, you can readily calculate how much braking force would be required to lift the rear wheel and start an endo. Once started the geometry changes making a full endo the outcome unless the braking force is reduced (not likely with a jammed wheel).

Without any data, I'll offer a crap shoot estimate that the required braking force would be someplace near 3/4s of the riders weight (depends on his riding posture). Once you have an estimate of the force needed, then the jammed wheel can be torqued to see if it is jammed tight enough to generate the required force or would turn.

All this assumes that we have a correct sequence of events. It's also possible that the crash (for whatever reason) caused the broken spoke. Something that would be more plausible if the evidence shows that the spoke couldn't have caused the crash.

This specific crash aside, the idea of crash due to spoke failure is worthy of further research. Starting with a general value for the force needed to cause an endo, the next step would be to determine how warped a wheel would have to be to bind enough to generate that force (on dry pavement), then finally, cut a spoke in a variety of wheels to see if they warp enough to be the issue.

Maybe Airburst, can write a proposal for a project and do it using her school's facilities.

If it turns out that a single spoke failure can cause an endo, it's worth considering whether this is an acceptable risk, and for who.