the only way a glock xd/m m&p will slamfire is a if the striker has debris holding it int he forward position or bent and stuck or broke(extremely unlikely) or the trigger safety has been disabled and the sear monkeyed with.
on a glock and M&P it is REQUIRED that the trigger be fully pulled to the rear since it is a double action only gun to cock the firing mechanism enough to have enough force to set a primer off. a glock or M&P cannot fire a primer unless the striker is pulled fully to the rear by the trigger. so even if the sear or trigger bar or other parts are modified incorrectly it still wont go boom unless the trigger is pulled.
im about 100% certain it was his booger-hook in the trigger guard that set it off