Must I stop writing the story after playfully cursing my friend, or can I salvage the situation?

The question is vague, but if your friend cursed the story, she erred. As for you, there is no blame upon you for completing its authorship as long as it does not include anything prohibited by Shariah. It is not forbidden to benefit from a cursed thing. The story of the woman who cursed her she-camel was a specific prohibition from the Prophet, may Allah's prayers and peace be upon him, against accompanying that cursed she-camel.