One should do. Use wireless at home and teach good habits. If there's really a need for more surfing when out and about, you can reevaluate.

10 GB

Depends on the forums, higher graphics are higher data drains.

Apps drain data fast. If she's playing something that will update in the background and notify her when the next step is ready, that's even more of a data drain.

Checking email doesn't use much data unless she's doing a lot of attachments.

How much is she allowed to use the tablet, how many apps will she be using and is there any way you can cut it off (ie. she hit 5 gb for the month at the end of the second week, no more tablet til next month)?

From 1 GB to 10 GB. For someone who likes to go on forums to chat and play app games. What would you get. She will also be checking emails.