My girlfriend trapped her thumbnail in a door and the nail has gone black.

It is growing out slowly, but she would like to know if there is anything she can do to speed it up?

Sorry to hear about your girlfriends thumb. The nail will grow at the normal pace so i'm afraid she will have to be patient. A well balanced diet wil help to support the repair but i am not aware of anything that will make it grow quicker. If the nail isnt lifiting she can always consider disguising it with a regular file and polish.