Lets start with she doesnt want it so therefor she doesnt want to spend money on it.

Let her know how much it is going to cost per month and yearly to use up time-gas-energy to run to where ever you will get the water as well as the containers and cost.

You can get a decent unit on ebay for less than 100.00

Now with the savings $$$ as well as with the time saved you can take her out more often !!! :lol