http://www.att.com/esupport/article.jsp?sid=53957&cv=820
most of your questions are answered in the "Important to know when you use call forwarding" section out going calls are allowed so I would think that texts and data are too.
Be sure to check into international plans as it says that minutes will be billed for the duration of the call (when I used call forwarding, 1 min was billed to the phone that I was using the forwarding feature on), just to make sure that you don't have to come back and post about outrageous international charges for calls that were forwarded. (I assume that she will be bringing her phone to Spain as you wanted to know about outgoing calls and texts). I hope this helps but still leaves you to check into a few things