Unfortunately, having an instructor provide a PADI cert number is apparently not enough. She had her cert number (I have it now as well...) and put it on our referral forms. I suppose that had we known to check this beforehand (meaning the weeks leading up to the trip, as opposed to the day of the trip), we could have contacted PADI to make sure everything was current and she'd be able to actually certify us. But I guess you live and learn.
If you had her cert number and name before you left for your trip, you could have called PADI (during normal business hours) and asked to verify her instructor's teaching status. Alternatively, your instructor at home could have pulled up the PADI Member's Area website and verified this information any time of the day or night for you.
I'm not blaming you -- most student divers have no clue that they can do this, nor that it can be a big headache if it turns out this way. Not to mention, we all like to assume the best in others, like they won't be misrepresenting their credentials...