Dehydration can cause organ failure and death if ignored.
There are 3 good reasons why your mother with Alzheimer's almost died of dehydration.
First, she is not taking in enough fluids.
If her caregivers are not adequately monitoring her fluid intake to ensure she takes in enough fluids, this will pose future risks to your mother's survival. You or whoever is her guardian, power of attorney, or conservator should request a review of her records. Do this during one of your visits so that you may review her records unannounced. (Sometimes, this is not possible without an appointment, but ask to look at her records anyway.)
Be sure you raise this issue at the next care plan meeting.
Finally, talk to other residents' families to see if there are other instances of dehydration among the residents.
A second reason why your mother gets dehydrated could be due to her medications.
For example, some heart medications act like a diuretic to prevent the weakened heart muscle from having to pump more liquids than absolutely necessary. These diuretics will cause dehydration if not managed properly.
Finally, your mother may be refusing to drink.
It's not uncommon to have a person with dementia refuse to drink water or even juice. The care facility's staff may explore different alternatives to liquids such as feeding her fruits and vegetables.
To know for sure, ask questions, observe, talk with other residents' families, with the director of nurses, and the care staff then document.