E.B.
Do you have wireless internet in your home? You can set the iPhone to use wifi and then it doesn't use data at all. And if she can access wifi at other places (friends' houses, Starbucks, etc) she won't be using data either.
I don't believe family lines can share data.
Here's a great presentation on how much 2 gb is, in terms of emails, web pages, etc.: http://www.washingtonpost.com/business/economy/how-much-i...