We have cruised on both Disney ships and loved them both . No mater what you do or what excursion you take I am sure you will have a wonderful time .The adult only restaurent was a nice added touch.
I haven't sailed the 7-day Disney cruise, but we did a 3-day last year. (My review is posted on cruisereviews.com.) Loved it, even without kids! I don't think you can go wrong, whatever you do. Enjoy!!
You may want to try disboards.com for specific disney info. Click on the cruiseline board about 1/2 way down on the left. They have lots of information and are very helpful. castawayclub.com can also provide information. We went on the 7 day Eastern in January, but did not have any teen agers. There is a club specifically for them as well as a sports area and pools. She should meet lots of new friends and have a great time. Have fun.