The cruise being on 1/6/08 you should not be in the plenty phase. (Normaly 70 days prior to the cruise) You should be able to change it without charge. IF you were all traveling in the same cabin, it may even be possible just to delete her from the cruise and not pay for her fare at all. But if they had their own cabin, then he would end up paying for extra and probably would be better off replacing the girlfriend as you are planning. But the 70 day mark is very close, so act today!
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, Celebrity Silhouette 2013