Three times to the range without understanding the part about keep your trigger finger off the trigger?
Are you telling me that you didn't violate rule #4 at least once in YOUR first 3, 5, or even TEN range sessions?
If I read the OP right, she was aware the firearm was loaded (all of them are, until locked open), so Rule #1 was being observed. She kept the muzzle in a place such that, if a discharge occurred, nothing of value was destroyed, so Rule #2 was being observed. She was trying to put the shotgun on "safe" before she moved, so safety was still her intention, even if it didn't get translated into her finger remaining outside the trigger guard.
It's ALWAYS a shattering experience to have a firearm fire before the handler is ready. But if it was simply DESTINED to happen, the lady seems to have done about as well as anyone COULD do, to minimize the after effects of such an event. She went into it, thinking to act safely, got fumble-fingered, and STILL weathered the event without harm to herself or others. She should be commended for doing everything ELSE right.