No, the cabin is probably listed as a 4 person max cabin and they would not let you book another into it. So, the option would be to pay additional money and book another cabin (for 2) but then the two passengers booked into that cabin would be paying full fare. So not only would it cost her friend more, it would cost you more too.
April is good spring break time, I would bet that there would be a lot more kids her age on that cruise then before. The odds are pretty good she could find someone to hang with.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007