So much fun! Tell her to definitely bring her friend along! It will make it so much more fun! I'm in a similar situation (20 and single) and my best friend is coming with me on the cruise this year and she came with me last year. There are a lot of adult activities and if she's shy or there are not to many people her age, bringing along a friend helps a lot. But fun? LOADS!!
Hope this helps!
~Melissa