[kwxza]

A good analogy she may grok easily could be comparing her plan data to her water usage and comparing her phone storage to the kitchen sink.

The sink can only hold so much water at once, but she can use more/less water each month than the sink can hold. If the sink is full, she has to drain some water to be able to add more. Also, if she uses too much water, she may be charged more than she was expecting.

I've explained storage vs data this way to a few people and it seems to have been useful.