Can I text from iPad with only an email address?
Off the top of my head, here are the settings that could affect that:


In Messages preferences, make sure that her email address is not only listed for both Send and Receive, but is also checked.
Make sure the checked email address is her iCloud address that was created when you/she set up iCloud (her_name@icloud.com).
At the top of the Messages prefs, make sure Messages is turned on.


On your side, if you're using a contact card to Message her, make sure you're using the "message" button, and not "mail". (Less likely, I know, but worth mentioning.)

Also, keep in mind that she can only send iCloud Messages with this setup. She'll need an iPhone if she wants to also use SMS.  (Apple uses the phone as the SMS gateway for other iDevices and Macs.)  So if you're sending SMS messages to her from a non-Apple phone (or if you've set your iPhone to only send SMS), she won't see it in Messages. 

That may not be comprehensive, but that's the stuff I can think of that I've seen muck up Messages before.  The "off switch" for Messages is probably the most common one.