PSKY,

Which ship is she have you made this reservation on? As has already been suggested some NCL ships have two bedroom suites that should solve your issue.

Is your suite designated as being a part of 'The Haven'? If that is the case then indeed non-Haven passengers cannot access The Haven (what really is a small First Class area on an otherwise one class vessel).

I do not know exactly why you cannot buy more than one room for a person, but it may be due to issues with passenger lists/souls on onboard (i.e. your daughter would be listed twice) but also due to the fact that onboard revenue would be offset by a berth being left unoccupied (although she is paying a supplement to cover that). Unfortunately you would find that most major cruise lines would be unhelpful in these situations.

Other than booking the two bedroom suite if the vessel you are on has it (I doubt it would be more expensive than listing a third passenger and an inside with a solo supplement) I may suggest finding/having a very knowledgeable and strong travel agent who knows everyone in the cruise lines, and has a great understanding of how things work would be of great help to get such things sorted.