Just to add into the mix....
I'm a nurse and worked for quite a while in a NICU in Nigeria. Some of the hospitals there are fab, absolutely top of the range, and you wouldn't neccesarily get any better care here than you would do there.
If this woman and her husband managed to get Visas here, and afford the flights (which are very expensive), I'd wager that they could have afforded to have the babies there. It's really hard to come here from Nigeria unless you have a lot of money/ contacts, so I'm not sure the 'well done, you've given your babies the best chance at life' comments are that relavent really.
Having said that, I agree she should be allowed to stay till her babies are well enough to travel. I also suspect there's a lot more to this story that meets the eye.