That she was so unresponsive especially for so long had me doubting anything postural as well, but that’s the best the MD who spent quite some time with her and her family came up with. What could cause her to go lights out that suddenly and severely? It almost certainly wasn’t a diabetic issue and there wasn’t any trauma.
That is a good question, but it's unanswerable because there are too many possible medical reasons for a person to become unconscious. That is why she deserved the kind of workup I suggested above.
Someone above mentioned hypercapnia, but that generally causes enough uncomfortable physical symptoms that a person snorkeling on the surface would likely abort long before becoming unconscious.
Then there is the complicating fact that she had had COVID, which can do deleterious things to various organ systems.
I hope someone figured it out and that she is OK.