Hypoglycaemia (to the point where blood sugar wasn't even registering) would indeed be dangerous if it wasn't treated promptly. (My blood sugar level frequently drops to well below 3, which technically means that I'm hypoglycaemic, but simply eating something soon pushes it up again).
I would assume that a low (or zero) level was recorded in response to excessive insulin, which would be quickly remedied by increasing your mother's blood sugar levels (either orally or, more likely, intravenously). Only a SUSTAINED very low blood sugar level would actually lead to medical harm.
Chris