It seems your daughter in law is suffering from DIABETES
INSIPIDUS( DI ).
Diabetes insipidus (DI) is an uncommon condition that occurs when the kidneys are unable to conserve water as they perform their function of filtering blood. The amount of water conserved is controlled by antidiuretic hormone (ADH), also called vasopressin
DI caused by a lack of ADH is called central diabetes insipidus. When DI is caused by a failure of the kidneys to respond to ADH, the condition is called nephrogenic diabetes INSIPIDUS.
Not drinking enough fluids can lead to the following complications:
Rapid heart rate
Unintentional weight loss
If treated properly , diabetes insipidus does not cause severe problems or reduce life expectancy.