Are you sure she didn't just pick up on the wrong BA12 being delayed and react too hastily (with BA also getting confused). If you search flight status on flights SIN-LON today, you get both today's flight she is on and yesterday's flight delayed into today. She may have jumped straight on the the latter (listed first).
It seems a bit unusual as I thought in these circumstances BA normally re-number the delayed flight to avoid confusion. Perhaps since only one BA12 will land at LHR today (as planned, but delayed) that isn't required.