This is interesting to me at the moment, the police have just taken a few of my images away in jpg format as evidence in a very serious case involving my girlfriend, I may have to give the officers dealing with the case a ring and mention this as they don't seem to know too much about image files, if I can supply them the nef's then they can prove they have not been tampered with.
Arkady is probably mentioning the Original Security Data implementations on the camera. These sign the images with a pre-determined key signature, in the same way that SSH encryption works.
Once the image is created, it is impossible to modify the image, without already having the private key. However, if you modify the image before the private key is put in, then the data can be modified, and re-coded.
Simply put, if you really wanted to, you could modify the sensor on the camera in order to re-play an image to the encoding program. It isn't easy, and to be honest, not worth the hassle.
Also, although the image itself would be indestinguishable from one which had been taken properly, an investigation of the camera itself (which if a court was concerned, they could easily request), would show that the modification had taken place.
As for your girlfriend...
It is unlikely that you will have already have put a coded key on your camera (I think you have to pay the manufacturers for a license anyway). So there is not too much benefit in sending them the raw. Keep it, they may ask for it later, but it is probably not storing the data required.
As for the OP's original intent. Changing the data in a raw picture, without the signed data, is possible. Assuming that it is not possible to fake a raw is wrong. A raw can relatively be changed, but would be noticed with a decent look. A more in-depth modification can be made which would make a more detailed investigation necessary. If you go the full hog, and look at evidence level confidence, then modification at the software and hardware level would be required.