ALB,
there are a number of reasons why your wife may have passed out and dehydration isn't one of them.
When you drink fluids and fluids exclusively then Blood Pressure goes down: this causes a lack of blood to the head when arising from a sitting or prone position. This can lead to dizziness and fainting.
It is a also a fallacy to believe that fluids taken in excess of thirst will do you any good except to overburden the body with the task of elimination.
For example, if we consume more calories than the body has need for and in opposition to hunger then we gain weight and the result is of course obesity.
If you over-breathe by deep-breathing then you will suffer the consequences of Hyperventilation.
The only safe and beneficial way of treating the elements vital for life is via the demands/needs of the body and actually listen to what it is telling you.