Hourly checks until under 250 or so seems pretty standard.
So, was the sugar much higher, combined with an overwhelmed ED, so maybe the nurse figured it would be safe to leave it for a while? Was she called away to a trauma or cardiac, and somebody else was supposed to track BGL in the meantime?
I agree this wasn't good, but it is a little too simple to indict the nurse as "inexperienced, incompetent, and / or uncaring". The description of shrugging the shoulders and answering 'it's been a while' could be an exhausted, embarrassed nurse who doesn't know what else to say just now.
Now, given that this is a risky way to do things, what professional channel did you all employ to follow up? Is this a charge-to-charge query, or manager-to-manager? Something not-too-drastic to clarify what really happened and how it needs to be addressed.
Ultimately it is the nurse's responsibility, but did a tech get assigned to track this and let her down while s/he was overwhelmed with more actively sick/injured patients? I've screwed up plenty over the last couple of years; but on one recent occasion I got chewed out by a doc for labs that didn't get sent off for cardiac ruleout over an hour before. I drew them, and the tech was supposed to send them off pronto. Well, I just told him 'yes, I screwed up' and got embarrassed; but I did not mention that the tech had the bloods and was supposed to send them. I knew the tech now knew the problem, and there was no point in making their night worse, too. They learned, and so did I, and we move on.
So, what really happened with this nurse? We don't know. Until then, I'll avoid marking her as 'incompetent' or worse.