Insulin shock

My mother-in-law was ill recently and did not eat very well for a day.  The next day she had a blood sugar reading of 70 before breakfast, so she took her glipizide as directed then ate breakfast. I stopped by right after that and she said she was feeling much better than the day before and I didn't need to come over for lunch. I did anyway, just 4 hours later and found her in a paralyzed state in her bed with her eyes open unable to respond to me. I called 911. I thought she had a stroke. They got there and checked her B/S. They had dropped to 47! She said she had fallen asleep and woke up unable to move, she doesn't know how long she laid there like that before I arrived. How much longer did she have before she would have gone into a coma? The ER doctor said a blood sugar of 40 is a coma, but I can't find any literature to back that up. She is only 65, but is exhibiting signs of Alzheimer's. I don't know how long I can leave her alone now. They took her off the glipizide, and now she is on a sliding scale of insulin only. I am just afraid she will forget to eat again!
Thanks