Legal Question in Immigration Law in Maryland

Re: Citizenship and rights for permanent resident under 21

At 18 years of age she's considered an adult in the US and can do as she pleases. Of course, her parents are free to kick her out of the house if they want but they can't force her against her will to return to Ghana.

Are you sure she's a "permanent resident" (i.e., someone who has a "green card")? If she is a permanent resident and was granted that status as a derivative of her father, then there is no problem -- she can stay here indefinitely in a lawful status and even apply for citizenship after she's held that status for 5 years. It doesn't matter how she became a permanent resident -- a permanent resident is a permanent resident.

However, the fact that her father works for an international relief organization doesn't mean that he is a permanent resident. There are other visas that could permit an indefinite stay but not technically be permanent residency. If she is a derivative on some non-immigrant visa (and not a permanent resident) then the parents could withdraw that visa as it relates to her, leaving her with no legal status in the US.

If you're sure she's a permanent resident, great -- she can do what she wants. If not, you may want to talk to an immigration attorney to go over what documentation she has so that her status can be determined.