If you do not depress the grip safety on an XD, the pistol will not fire even if the trigger is pulled nor can the slide be cycled. If the shooters grip failed to depress the GS, the gun would simply not fire. As such, I cannot fathom how the GS could induce a stovepipe. That said, anything is possible.
Could be a matter of grip strength, technique, or a combination of both. Could even be that the pistol seemingly functioned fine for the other 90 rounds, but in actuality it's right on the edge of failure due to strength, technique, or even a maintenance issue. Can't say for sure via an internet diagnosis, but she's certainly not the first person to ever experience malfunctions shooting from retention.
If someone spent five minutes with her to nail down the technique, plus hit the pistol with some extra "just because" drops of lube, I'd be willing to bet she could run that drill without fail moving forward.
Strength is always a factor, and one if it's biggest advantages is masking less that stellar technique. Really strong people can mash the crap out of a trigger with less effect on accuracy than those with less strength. Also, it usually only takes a minute to teach someone that appears physically incapable to lock the slide back on a semi auto by showing them a better technique.