She is living in a shelter and, along with 9,000 other people in the shelter, she needs water to drink.
Suppose the water was $5 per case at the time of the hurricane. This will be true of the other 9,000 people in the shelter as well.
It also means that more bottled water must be produced, since the demand for bottled water in other areas not hit by the hurricane will not be reduced.
Now, we could rely upon the good graces of those who own the bottled water companies to produce more bottled water at the same price as they were before the hurricane.
And as the price rises, those who value water less than your mother will reduce how much they demand, leaving your mother with the chance to buy the water she needs.
Trying to solve her problem with a price gouging law will result in most of the people in her shelter not getting water. The solution is instead to have people donate money to agencies that buy the water at $20 per case and give it to your mom.
Many corporations, aware of these economic realities, have taken the lead in relief efforts. More water will be available to people at the shelter using this approach than by limiting the availability of goods and services through price gouging laws.